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
Merge pull request #26 from astrodsg/master

Enhance tags