Added locator support to frame_to_be_available_and_switch_to_it().

The Python bindings currently only allow frame ID or frame index to be passed to expected_conditions.frame_to_be_available_and_switch_to_it(). It should also be able to receive a locator, like most (all?) of the other expected_condition methods.

This also brings the Python API in line with the Java API, which has two
method signatures for frameToBeAvailableAndSwitchToIt():

  * public static ExpectedCondition<WebDriver> frameToBeAvailableAndSwitchToIt(java.lang.String frameLocator)

  * public static ExpectedCondition<WebDriver> frameToBeAvailableAndSwitchToIt(By locator)

I have signed the CLA.