Change the __isrtl__ message keyto isrtl

A message key with such underscores is problematic for translatewiki.
Because translatewiki is a wiki, and message keys are page title,
underscores are handled in a tricky way, so it's better not to have them
in messages.
Merge pull request #97 from amire80/isrtl

Change the __isrtl__ message keyto isrtl