`fbnn` documentation with torch-dokx and script.

Summary: Just like the `fbcunn` diff that preceded this one.

Test Plan: See generated documentation at http://home.fburl.com/~tulloch/fbnn/

Reviewed By: soumith@fb.com

Subscribers: jhj

FB internal diff: D1740702

Tasks: 5822357

Signature: t1:1740702:1418682841:c3c13064e2f52d3cde8203ea4bff8ab0c83fa36a