Enhanced tags

The reference I found for ds9 region files [ds9 regions](http://hea-www.harvard.edu/RD/ds9/ref/region.html) states that you can have multiple `tag` keywords in the comments. This change will collect all the `tag` keywords into a set object which can then be used later by the user for grouping together regions
Enhanced tags

Enhance the capability to add multiple tags
updated ShapeList doc string
change 'import pyregion' for sphinx compilation
fix syntax
get_mask bug fix: header<->origin

Fixes a bug in 'get_mask' function where a header was passed to
'as_region_filter' as 'origin' argument
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]
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
Merge pull request #26 from astrodsg/master

Enhance tags
update CHANGELOG for PR #26. Note that PR #26 will change the type of 'tag' attrubite from a string to a list of strings.
Merge pull request #32 from mcara/get_mask-bug-fix

get_mask bug fix: header<->origin
change the signature of ShapeList.__init__
implement projection and segment shapes
Update version requirements to allow pyparsing 2.0.1 for py 2.6,2.7
Merge pull request #36 from taldcroft/pyparsing-version

Update version requirements to allow pyparsing 2.0.1 for py 2.6,2.7
Fix setuptool configuation problem with Python 3. The issue is that it downloads setuptools and runs 2to3 on it. It does not work. There are packages for setuptools in python 3 so this should not be required
Merge pull request #37 from mboquien/master

Fix setuptool configuation problem with Python 3. The issue is that it d...
Merge pull request #33 from mcara/write-fixes-enhancements

Fixes and enhancements to ShapeList.write()
added .write function to ShapeList