Keys.META is now regarded as modifier key in SingleKeyAction

Keys.META, Keys.COMMAND, Keys.LEFT_ALT, Keys.LEFT_CONTROL and
Keys.LEFT_SHIFT are now also regarded as modifier keys in
SingleKeyAction (because of their aliases).  This fixes the issue
reported in

    https://code.google.com/p/selenium/issues/detail?id=4843

Signed-off-by: Andreas Tolfsen <ato@mozilla.com>
Updated AUTHORS