NetFileCache and LocalRepo

- LocalRepo - use a local folder as a CKAN repo
- Replaced Cache with NetFileCache, now everything is cached by url according to #111 
- No tests yet :(
- Doesn't get filename from HTTP headers yet
It looks like our KSP class already clears the cache of files: https://github.com/KSP-CKAN/CKAN/blob/master/CKAN/CKAN/KSP.cs#L180