Added locator support to frame_to_be_available_and_switch_to_it().

This 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)

Signed-off-by: Luke Inman-Semerau <luke.semerau@gmail.com>