Marlin tools

Implemented two tools (and a trivial library) in Ruby for working with Marlin instances.

* `verify-integrity.rb` ensures both that the files listed by `/all` are on disk in the expected place and that the SHA-1 and filesize in redis match those of the files on disk.
* `verify-consistency.rb` ensures that the list returned by `/all` on two (or more) different Marlin endpoints are the same, and that the file attributes returned by `/<filename>/all` match on all the provided endpoints.

Check out the README for usage information. Please let me know if you run into issues while testing prior to merging.

Cheers,
Thom