mod_roster: Add a "roster-get" event

[Alexander 'PapaTutuWawa']

Hello everyone,

quite some time ago I have announced that I am working on modules
that implement the MIX family of protocols for prosody. As I am
currently working on making these modules upstreamable, I wanted
to start with something simple:

This patch adds an event to mod_roster that is fired before a
roster-get query is answered. This is required by, for example, the
MIX-PAM XEP since it needs to annotate the roster-get response.

Regards,
Alexander "PapaTutuWawa"