Fixed some bugs with the DroneController class. Things working as expected.
flownav,common: a few fixes and improvements
Added functionality for keeping a history of keypoints including opencv
keypoints, descriptors, and corresponding images

framebuffer: added internal buffers to the buffer classes that keeps
track of previous frames and their times. However ROSCam has NOT been
tested.

common: added keypointhistory class to assist in tracking keypoints;

flownav: modified to track expanding keypoints over multiple
non-adjacent framesl