Fix open email link for Google Apps domains

This is a proposal to (partially) fix issue #8

It replaces the `/mail?` part of the link with `/a/<domain>?` (same as before) but also adds the message id in a hash as suggested by @johndbritton.

This works if logged in to **one** account per domain only. It can not discern between multiple accounts on the same domain (as far as I know).

Hope this helps, please note that this is my first contribution to an Objective-C projects so although I tried my best it's quite possible that it's not very good.