Add signal functionality to manage events (after_token_refresh)

[Kyria]

Related to #46 (and may fix it)

This adds a base functionality to trigger alarms/signals (or whatever the name you give them) in pycrest.
Basically, you have instances of Signal, each one represents a specific event. These signals, when triggered (using Signal.send(**kwargs) or Signal.send_robust(**kwargs) if you want to silence exception (still logged using logger)), will call all receivers they registered and give them the same arguments.

A receiver is just any callable object you my have (function, class methods, class with __call__()).

The doc has also been updated to show how to use signals in the code (import signal, add/remove receivers)

Currently the only event added is the after_token_refresh signal, fired each time AuthedConnection.refresh() is called with these arguments :

Arguments	Type	Description
access_token	String	The new access token used to log in
refresh_token	String	The refresh token used to refresh
expires_in	int	The token validity time in seconds

This a really lightweight inspiration from the django's signal that gives the advantage the "core code" only need to know which events he wants to fire, without dealing with list of receivers, etc.

On a side note, and not directly related to this PR main subject, I splitted the tests, moving cache tests and event tests in their own files (and fixing a few pep issues at the same time)

[coveralls]

Coverage decreased (-0.3%) to 99.704% when pulling 30f69ee on Kyria:events into 26d72a4 on pycrest:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1cbd236 on Kyria:events into 26d72a4 on pycrest:master.