replace cElementTree with ElementTree

[knight-of-ni]

fixes #203

This fixes the most immediate need by replacing cElementTree with ElementTree in these files:

ouimeaux/device/api/service.py
ouimeaux/device/maker.py
ouimeaux/device/bridge.py
ouimeaux/subscribe.py

However, a better fix might be to implement a series of try/except, similar to what has already been done here:
https://github.com/iancmcc/ouimeaux/blob/develop/ouimeaux/device/api/xsd/service.py#L27
https://github.com/iancmcc/ouimeaux/blob/develop/ouimeaux/device/api/xsd/device.py#L27

I'll leave that up to the author to decide.

[iancmcc]

LGTM. I need to go through and clean up those imports to do it centrally, but in the meantime this is perfectly fine.