Enable connections to secure MQTT brokers

[ldstein]

This PR adds support for connecting to a MQTT broker which requires SSL/TLS authentication.

Also a bump of async_bench to allow dev-dependencies to install in a Windows environment.

Pretty much addresses issue #78

[coveralls]

Coverage decreased (-0.07%) when pulling 7b4da72 on ldstein:master into 1a7f48d on mcollina:master.

[mcollina]

@ldstein I'm slightly against this. MQTT.js is going to deprecate the old SSL API in v0.4.0 and remove 'keyPath' and 'certPath' in favor of buffers.
mqttjs/MQTT.js#145
That PR is missing the 'client' part -- the server has already been done. If you help with that, we can merge and release MQTT.js 0.4.0.

@adamvr is it ok for you?

@ldstein Can you please add test for this in any case?

[mcollina]

Ouch, this is really really old. However, I should get it in. Anybody that would like to update it to v2?

This would need to be updated to MQTT.js v1.x.

[ldstein]

Happy 2019. I believe this PR can be safely closed :)

For anyone reading this, below is an example to connect to a secure MQTT broker configured with a certificate derived from a self-signed CA (tested with ascoltatori 4.3.0):

const mqtt        = require('mqtt');
const ascoltatori = require('ascoltatori');
const fs          = require('fs');

const settings = 
{
    type: 'mqtt',
    json: true,
    mqtt: mqtt,
    url : 'mqtts://localhost:8883',
    // Plus any additional required TLS configuration
    // as described by https://nodejs.org/api/tls.html#tls_tls_createsecurecontext_options
    ca  : [fs.readFileSync('certs/ca.crt')]
};

ascoltatori.build(settings, function(err, ascoltatore)
{
    if (err)
        return console.error(err);

    ascoltatore.subscribe('test', console.log);
    ascoltatore.publish  ('test', 'Hello world');
});