Add support for validating rusage element, and Hashdeep (md5deep, etc.)

Merging this pull (really, [this commit](https://github.com/ajnelson/dfxml_schema/commit/d1b5a5846258c8f3e3085f85516afd1cf1034254)) will cause the `rusage` and `error` elements to validate.  This is the last needed for Hashdeep to validate.

I've gotten one nod for this to go in.  I will merge it right after rfc3.