bridge-move/example.py: Add a python example for moving between bridges

This patch adds an example that shows moving a channel between bridges.
It uses a holding bridge as a holding area for channels initiating a
dial. When the outbound channel answers, the inbound channel is moved
from the holding bridge to a mixing bridge with the outbound channel.
Merge pull request #6 from matt-jordan/bridge-move-python

bridge-move/example.py: Add a python example for moving between bridges