Better translation UI

This pull request bring in #23 and #15 plus an addition to remove existing translations using javascript (see 3f67cfab5a21158f5242424d8b8c842f2ec95017).

With this the before_filter implemented in #14 is not needed anymore.

I've also relaxed globalize dependency to `~> 3.0.4` to allow usage with rails 3.2.x

If this gets merged #10, #19, #20, #28 and #29 may be closed since all of them will be addressed.

@stefanoverna, @dtengeri, @sepastian let me know if there is something else that should be done.