Removed auto-setting of shutter_speed to 0

This is to permit the class to be constructed and used successfully in
the case the user is running down-level firmware (in which case setting
shutter_speed will fail)
Bumped revision and updated changelog

In preparation for release of 0.6
More extensive tests

New tests for the exc unit (trivial) and started writing mock tests for
the camera module to bring up the coverage