Add #files to maid/tools
Add #size_of and #checksum_for helpers to the Maid DSL
Add #dupes_in to the Maid DSL.

See #15 and https://gist.github.com/4321983

No spec, because I wasn't sure how functional or mocked this one should be :-/
Merge pull request #92 from bobthecow/feature/dupes

Implement dupe detection