Enable "snoop" in Trigger by default

When no snooping is specified, the default is to enable it. This removes
some cruft from the Bugzilla plugin and makes the code look neater.
Improve presence handling, add composing notifications

Presence handling now uses constants, to avoid sending malformed stanzas to
the server. Also, druid.Request now handles changing states for composition
notifications:

- Whan a trigger is activated (matched), the status is set to "active"
  by Request.start()
- If an operation is going to take a while, triggers should call
  Request.replyLater(), which will set the status to "composing"
- Once a trigger replies, it should call Request.end() to set the
  state back to "inactive"