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]
Merge pull request #33 from mcara/write-fixes-enhancements

Fixes and enhancements to ShapeList.write()