Write proper unittests for library

[deepbrook]

the current testing setup is still identical to the originally forked project's test-suite. As it is no longer up-to-date with this fork, it needs an update.

This should be done using a unittest suite, using pytest, and an integration test suite (similar to the legacy test code).

UnitTest cases :

• assert robustness of system message handling (messages sent by Pusher to us should be handled correctly).
• URLs for websocket connection is built correctly.
• connection state updates are handled correctly.

Integration tests:

• connect to pusher using credentials.
• subscribing to a channel works.
• unsubscribing from a channel works.
• pings invoke a pong response.
• registering a callback works for single event per channel.
• registering callbacks for multiple events per channel works.