Bug fixes and enhancements in 'write',rot_wrt_axis

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 CS [enhancement];

4. 'pyregion' computes the rotation angle using X-axis instead of
adopted convention of using Y-axis (at least for HST images). This
produces regions that are rotated compared to the regions produced by
DS9. 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 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' [bug fix].