Refactored the configuration of sessions

The configuration now makes sessions a first-class citizen instead of
being centered around drivers. This allows defining several session based
on the same driver type.
Instead of allowing other extensions to register their own sessions to add
new drivers, they can now register a DriverFactory in the MinkExtension
during the extension initialization to make a new driver type available.
Added some validation of the default session and javascript session
Removed unused use statements
Added a shortcut syntax for the configuration of sessions
Merge pull request #121 from stof/driver_factory

Refactored the configuration of sessions