Added function to detect cameras.
Added cooling property to Canon for compatibility.  It reads True or False as if it were a cooled camera, but does nothing.
Merge branch 'develop' of https://github.com/panoptes/POCS into develop
* Renamed classes so that AbstractCamera is the class from which specific cameras inherit.
* Renamed CanonDSLR class to Camera so that the import will always import a Camera object, but from a module specific to the type/model.
* Added generic set(), get(), and command() methods for interacting with gphoto.
* Modified many CanonDSLR methods to work with new set(), get() and command() methods.