Review of PR : FAST and ORB

Hi all, The FAST and ORB pull request<https://github.com/scikit-image/scikit-image/pull/834>needs a final round of review. Johannes is done with some crucial design changes on the original PR<https://github.com/scikit-image/scikit-image/pull/699>. Please review it if you have some time. This might sound pretty lame, but merging this PR will motivate me to complete the work on FREAK descriptor<https://github.com/scikit-image/scikit-image/pull/729>. Thank you. Regards, Ankit.