Make seeing status updates configurable

With large rosters, the communication signal can get lost in the noise of
joins/parts/aways ( as pointed out in agl/xmpp-client#19 ). This commit adds a
/statusupdates command that toggles seeing these updates.

It is done in such a way that the default behavior is unchanged, so users who
don't care won't see any difference.
Merge pull request #24 from rmoore/hide_status_changes

Make seeing status updates configurable