Implement a form of the Scope Manager RFC

See https://github.com/opentracing/specification/blob/f7ca62c9/rfc/scope_manager.md
However, use continuation passing style which better fits JS and available APIs.

Implementations:
* AsyncHookSpanManager - uses Node.js async_hooks
* AsyncWrapSpanManager - uses async-hook-jl which uses Node.js AsyncWrap
* ZoneSpanManager - uses Zone.js

.travis.yml

@@ -5,11 +5,17 @@ node_js:
 
 env:
   matrix:
-  - TEST_NODE_VERSION=0.10
+  - TEST_NODE_VERSION=0.10 TEST_ZONE_JS=true
   - TEST_NODE_VERSION=0.12
+  - TEST_NODE_VERSION=0.12 TEST_ZONE_JS=true
   - TEST_NODE_VERSION=4
+  - TEST_NODE_VERSION=4 TEST_ZONE_JS=true
   - TEST_NODE_VERSION=6
-  - TEST_NODE_VERSION=node
+  - TEST_NODE_VERSION=6 TEST_ZONE_JS=true
+  - TEST_NODE_VERSION=8
+  - TEST_NODE_VERSION=8 TEST_ZONE_JS=true
+  - TEST_NODE_VERSION=10
+  - TEST_NODE_VERSION=10 TEST_ZONE_JS=true
   global:
   - secure: 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
   - secure: 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

README.md

@@ -105,6 +105,62 @@ const tracer = opentracing.globalTracer();
 
 Note: `globalTracer()` returns a wrapper on the actual tracer object. This is done for the convenience of use as it ensures that the function will always return a non-null object.  This can be helpful in cases where it is difficult or impossible to know precisely when `initGlobalTracer` is called (for example, when writing a utility library that does not control the initialization process).  For more precise control, individual `Tracer` objects can be used instead of the global tracer.
 
+### Span manager (experimental)
+
+This library supports span activation in the spirit of the [Scope Manager](https://github.com/opentracing/specification/blob/f7ca62c9/rfc/scope_manager.md) draft RFC.
+
+The intention is that it remains active within the executed function and its transitive call graph, including both synchronous and asynchronous calls.
+
+```js
+tracer.activeSpan() // null
+tracer.activate(span, () => {
+    tracer.activeSpan() // span
+    setTimeout(() => {
+        tracer.activeSpan() // span
+    }, 0);
+    (async () => {
+        tracer.activeSpan() // span
+    })();
+});
+tracer.activeSpan() // null
+```
+
+JavaScript support for continuation local storage is a patchwork.
+This library includes simple wrappers around a few common APIs for continuation tracing.
+
+**ZoneSpanManger**
+
+This uses [Zone.js](https://github.com/angular/zone.js). (See the associated [Zone](https://github.com/domenic/zones) ES TC39 proposal.)
+It includes support for most common APIs: Web, Node.js, Electron, RxJS. Async syntax (async/await) is not supported.
+I.e. you should target ES 2015 or earlier syntax.
+
+```js
+import 'zone.js';
+import { ZoneSpanManager } from 'opentracing/zone_span_manager';
+
+new ZoneSpanManager();
+```
+
+**AsyncHookSpanManager**
+
+This uses the Node.js [async_hooks](https://nodejs.org/api/async_hooks.html) API, available in Node.js 8.1.x+.
+
+```js
+import { AsyncHookSpanManager } from 'opentracing/async_hook_span_manager';
+
+new AsyncHookSpanManager();
+```
+
+**AsyncWrapSpanManager**
+
+This uses [async-hook-jl](https://github.com/Jeff-Lewis/async-hook-jl) which patches the Node.js AsyncWrap API, available in Node.js 0.11.x - 7.x.x. Though without futher transpilation async-hook-jl requires Node.js 4.x.x+.
+
+```js
+import { AsyncWrapSpanManager } from 'opentracing/async_wrap_span_manager';
+
+new AsyncWrapSpanManager();
+```
+
 ## API Documentation
 
 There is a hosted copy of the current generated [ESDoc API Documentation here](https://opentracing-javascript.surge.sh).