Svnfs

This isn't ready for merge just yet. Making the PR now to get some feedback. 

@techhat since you mentioned it in #7781, this code could use some testing and feedback. I haven't had a chance to verify that the code actually works :)

Also, this current is using [pysvn](http://pysvn.tigris.org/). Which does not appear to be available on pypi. So I am not sure how I feel about making it a dependency.