[voicecall] Improvements to ofono plugin.
Merge pull request #24 from martinjones/master

[voicecall] Improvements to ofono plugin.
WIP: Replace StreamChannelListener in commhistory-daemon by logging events in voicecall.

This involves a lot less fiddling to get properties out of a telepathy stream,
and generally Just Works.

The one part of missing functionality is that there is no periodic commit of
event modifications every 5 minutes, but if desired, that is easy to restore.
[voicecall] Warn if default provider is requested and none can be found.