Fix toLowerCase error on real requests

See #41 for more details

Support of superagent v2 and strict match of requests

• Better support of superagent v2
• Support strict match of requests with query parameters

Remove a warning when using with superagent v2

• Add a check for deprecated method setStatusProperties in superagent v2
• Small code improvements

Add logging and improve 'end' method

• Add logging functionalities
• 'end' callback is now called once, even in error case
• 'end' method returns now the request object

Add support of 'abort' method

It is now possible to call abort on the request.

Add support for chained calls to 'set' method

It is now possible to set() headers one by one and chain the set() calls.

Disable mock

Add a method to unset mock and fix call to real 'set' method.

Differentiate callbacks depending on request method

Add explicit support for request methods (GET, POST, DELETE ...).

Use superagent response error

Apply "sugar" behavior on error responses.

Support headers setting

Get access to the headers set through superagent.