Bug fixes, enhancements in 'write', rot axis selection

1. 'pyregion.write' crashes when saving regions without comments [bug fix];

2. 'pyregion.write' converts exclude ("-") DS9 regions to regular ("+") region when saving region files [bug fix];

3. 'pyregion.write' cannot save region files that contain regions with mixed Coordinate Systems (e.g., image, sky, etc.) [enhancement];

4. 'pyregion' computes the image rotation angle using as the angle between North and X-axis instead of typically adopted convention of using Y-axis (at least for HST images) for image orientation computations. This produces regions that are rotated compared to the regions produced by DS9 when image axes are non-orthogonal. This was fixed by introducing additional parameter 'rot_wrt_axis' (can be 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. [enhancement]

5. Fixed a bug in 'get_mask' function where a header was passed to 'as_region_filter' as 'origin' argument [bug fix].