Removed keysSpecial() method and added a constant-based solution instead.

Otherwise the IE driver doesn't work - it doesn't follows the recommendation of how sticky keys should work:
Recommended: they are sticky until unpressed
Implemented: they are sticky within a single keys() call
Restored deprecated PHPUnit_Extensions_Selenium2TestCase::keysSpecial() for maintaining BC
Merge pull request #227 from zerkms/special-keys-ie

The current keysSpecial() solution doesn't work with IE driver well