Previously a new call id was generated for INVITE and CANCEL.
Now the id generated during the initial INVITE will be used for
corresponding CANCEL events. Also, adding the ability to
trigger a call cancel via the poltergeist update api.

set up async context for all poltergeist calls
utils lib now has async wrapper for event handlers

If we do not reuse previous presences we lose avatar and name and people joining after the poltergeist creation will not be updated with those values.

Adds http method to remove poltergeists. Adds configurable timeout after which poltergeists automatically are removed from the room.

Thanks to Matthew Wild for the initial help of creating these.
Module with REST interface to create poltergeist participants and change their statuses.
When user with same id joins the room, the poltergeist is removed.  We also make sure that that user uses same username when authenticates. This way we are sure that user will join the room with the same nick as the poltergeist.