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
Uh oh! That's going to give us lots of pain on multi-user systems! If `/tmp/ckan_temp` has been created by one user, then it can't be used by another.

What's more, it may even expose us to symlink attacks. If an attacker places a symlink in `/tmp/ckan_temp` to point to somewhere else on the system, we'll go start writing files there.

While neither of these are going to keep me awake at night, they're both easily fixed. While we don't have `mkdtemp` available to us in C#, we can use `GetTempFileName` to create a temp file that we know did not exist previously, and then delete that and create a directory in its steed.