Merge pull request #9 from ahojnnes/orb

Hessian matrix and structure tensor
Making the example more explicit
Inserting corners in the corner_orientaions example
Implementing keypoints_orb without using Harris response
Returning best_keypoints
Docstrings; Stacking lists; removing redundancy in corner_orientations
Adding explicit test for corner_orientations
Implementing descriptor_orb; for loop in Cython
Returning keypoints; Adding docstrings
Minor doc change
Better naming of arguments
Writing better Cython code
Stacking lists in descriptos_orb
Adding example in descriptor_orb
Converting descriptors dtype to bool
Minor optimizations; Renaming variables; Docstring corrections
Adding square examples in keypoints_orb, descriptor_orb
Introducing the _prepare_grayscale_input_2D function in feature.util
Making pos0 and pos1 as global variables
Fixing the import issue