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 verify that only the expected elements have been selected, using for example `text()` not only that some elements have been selected. It's probably also worth merging this test with the one above (`find by a By object selector`)