Add default implementation of Conference interface
Add default implementation of Conference interface

The connection managers needs to properly populate:
 * self._conference_channels
 * self._conference_initial_channels
 * self._conference_initial_invitees
 * self._conference_invitation_message
 * self._conference_original_channels

max-posedon/telepathy-python,detrout/telepathy-python,max-posedon/telepathy-python,detrout/telepathy-python,freedesktop-unofficial-mirror/telepathy__telepathy-python,freedesktop-unofficial-mirror/telepathy__telepathy-python