initial gphoto2 support

This is basic gphoto2 support, based on the piggyphoto library that you suggested (with various bugfixes).  I've only tested this using the command-line mode so far, and only against Canon T2i & 5Dm2 cameras.

I'm completely new to python, so please let me if any of this is non-idiomatic, etc.

I'm also not sure if this should be on a branch instead of master and what other requirements you might have for submissions, so just let me know..

thanks!