HoG orientation

Just added a PR (#715<https://github.com/scikit-image/scikit-image/issues/715>) to allow computation of signed orientations in HoG. Minor feature, really, and perhaps not all that useful given that there is a substantial revision proposed to the HoG code in PR #703. All the PR proposes to do is to pull out the 180 degree constant into a separate variable and initialize it based on whether orientations should be signed or not.