Fixes and enhancements to ShapeList.write()

The following issues are being addressed in the proposed changes:

1. `pyregion.ShapeList.write` crashes when saving regions without comments [bug fix];

2.  `pyregion.ShapeList.write` converts _exclude_ ("-") ``DS9`` regions to regular ("+") region when saving region files [bug fix];

3.  `pyregion.ShapeList.write` cannot save region files that contain regions with mixed Coordinate Systems (e.g., image, sky, etc.) Proposed here changes will allow saving region lists with mixed coordinate systems. [enhancement]

This replaces a part of the PR #29.