Allow selection of the axis for angle measurement to the North

`pyregion` computes the image rotation angle using as the angle between
North and X-axis instead of typically adopted convention of using
Y-axis for image orientation computations. This produces regions that
are rotated compared to the regions produced by DS9 when image axes are
non-orthogonal (at least for HST images). The proposed here fix
consists in introducing an additional parameter `rot_wrt_axis` (can
take values 1 or 2) to let users select what axis should be used for
computing image rotations (-> rotations of regions). For example, by
setting `rot_wrt_axis`=2 `pyregion` now will use `rot2` (angle between
Y-axis and North) instead of `rot1` (in ``wcs_converter.py``) in
computing region rotations.
Merge pull request #34 from mcara/image-rot2North

Allow selection of the axis for angle measurement to the North