Redis updates WIP

Changes redis backend to bring it up to date with current libgit2 and make it a bit better in the proces. 

## Todo

- [x] update redis backend
- [x] prefix odb keys to enable multiple repos in one redis
- [x] make the top odb key prefix dynamic (currently 'rugged')
- [x] add a refdb backend
- [x] use [giterr_set](http://libgit2.github.com/libgit2/#HEAD/group/giterr/giterr_set_str) where possible