"Invite" StatusNet message

Ok, I've hacked in support for this, not yet used.

pgettext and npgettext are implemented as macros (wtf?!) in gettext.h and aren't provided in PHP's gettext module, but it was easy to add compatible wrappers and I've confirmed they actually seem to work. :)

Warning: xgettext by default doesn't recognize those as keywords for PHP source files, so these parameters must be added to the xgettext command-line when rebuilding the .po template:

   --keyword="pgettext:1c,2"
   --keyword="npgettext:1c,2,3"

Something like this, if consistently used, should do the trick: pgettext("tab", "Login") vs pgettext("button", "Login")

Anything else we should add to the list before we go contextualizing a bunch of messages?