Various bits of tidying... trying to make RepoRewriter tests more terse

RepoRewriter now returns a complete  map of modifed objectIds
Add basic support for deleting folders

This is not really the full implementation of issue #12 ("Ability to
remove directories"), because that issue actually requires
path-dependant pruning. This is just simple filtering based on
folder-name, but it should help this guy:

http://stackoverflow.com/q/16821649/438886

..and also Francis with https://github.com/janua/premiumparking

To create the nasty folder-named-'.git' repo, this is the code I used:

https://gist.github.com/rtyley/5673862