Remember all sessions, created from BrowserConfiguration

[aik099]

Make BrowserConfiguration::createSession method to remember each created session somewhere in central place (shared between browser configurations).

Then via register_shutdown_function ensure, that all sessions are stopped. This would save the day when #5 will be implemented, because we'll have 1 session per browser which is only reset (not stopped) in each test/test case.

[aik099]

Session creation has been moved to SessionFactory class. In reality no session should be running when the register_shutdown_function will be called.