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
This test is unnecessary, there's no point in testing that `$("#a")."$findMethod"(...)` is equivalent to `$(By.id("a"))."$findMethod"(...)` because a `Navigator` will always be a `Navigator` and we have already verified that using `$()` together with `By` returns the expected results.