Add beginnings of an communication interfaces
Bring USB functions into PTPUSB class, add IPTPComm interface, and modify build file to build PTPUSB.  Still need to integrate IPTPComm into CameraBase
Tweak CameraBase and CHDKCamera to use an IPTPComm
Required some additional functionality in IPTPComm -- is_open.  Now, we can ensure we detect if we can communicate before attempting to.
Make PTPUSB initialize and exit libusb
Include interface and implementation in libptp++.hpp