Merge pull request #154 from tiborsimko/mu4e-fix-q-binding-in-about-buffer

* mu4e: fix `q' binding in About buffer
Fixed bindings of entries in mu4e's header menu.
* mu4e: update documentation
* mu4e: don't use beginning-of-buffer non-interactively (fixes compilation warning)
Set mu4e-headers-view-win to nil when closing message view.
Merge remote-tracking branch 'upstream/master'