FrameListener pullrequest

I moved the definition of the FrameMap to the frame_listener.hpp and extended to FrameListener to have prototypes of all needed methods, that are implemented in the actual FrameListener implementation. Also I added a static method for creating a new FrameListener.
With this changes the projects that are using libfreenect2 only have to include the libfreenect2.hpp and the frame_listener.hpp. They do not have to include the frame_listener_impl.h anymore.