Add: method getKeypointsIndices to get keypoints indices in input

Provide a method to return found keypoints indices in the input. Not
all detectors would populate the keypoints_indices_ so you need to
check for emptiness befor use in set****Indices methods.
Add: example for getKeypointsIndices method
Merge pull request #318 from nizar-sallem/kp_indices

keypoints indices feature