Refactored the handling of Selenium2 capabilities

Saucelabs-specific capabilities are removed from the Selenium2Factory in
favor of the SaucelabsFactory.
The SaucelabsFactory now supports all normal capabilities (inherited from
the Selenium2Factory) and the list of Saucelabs-specific ones is
completed.
Added a way to specify custom capabilities for Selenium2

This allows configuring capabilities which are specific to other drivers
or custom implementations.
Fixes #129
Fixes #125
Refs #135
Added a factory configuring the Selenium2Driver for BrowserStack

Closes #103
Replaces #133
Renamed the SauceLabsFactory for consistent casing
Added some missing SauceLabs capabilities

Closes #126
Merge pull request #137 from stof/saucelabs_capabilities

Refactored the handling of Selenium2 capabilities