Add nemo-gtkhash

This is a fork from 0.7.0 upstream, modified to install a standalone nemo-gtkhash
extension. 0.7.0 includes a Nemo extension by default - this version will be superceded
automatically when 0.7.0 is included upstream.
Merge pull request #35 from wrouesnel/nemo-gtkhash

Add nemo-gtkhash