Implement dupe detection

This pull request adds `#files`, `#size_of`, `#checksum_for` and `#dupes_in` to the Maid DSL.

There are specs for all except `#dupes_in`. It works though :)

Fixes #15

[See this gist](https://gist.github.com/4321983#file-rules-rb-L7-L32).