Selenium By locator support in Geb

The following pull request adds support of using Selenium By class locating strategy in Geb.

This is based on the following geb-users discussion:

http://markmail.org/thread/qx3yjz4hwcce5oy2
Please mention that users should prefer the idiomatic Geb way of selecting elements using text css selector and that his has been introduced for convenience only. Please mention that apart from xpath support there is always an idiomatic selector equivalent of what can be achieved using `By` selectors .