Add svn_pillar - pillars stored in SVN

A functionally identical port of git_pillar to SVN.  The code is based on git_pillar.py, but with some restructuring, simplification, and clean up.

I'm not a Python programmer, so I'm not sure about a couple of things.  git_pillar was swallowing exceptions, so I did the same.  I normally would log and rethrow.

I did minimal testing.  I know it works well with base/trunk, but only tested a branch once.