Support for Discount 2.0.7's footnotes option

Hi Ryan

This pull request adds support for the :footnotes option to RDiscount#new, for Discount's new footnotes feature.

This is my first pull request and I'm not sure of the best way to handle the changes to the discount submodule. All I've done in the submodule is update the discount sources to tag v2.0.7 from Orc/discount, but this pull request (for **r**discount) doesn't include the updated subproject revision; I'd have to submit a request for that (**d**iscount), then wait for you to pull, *then* submit this pull request (**r**discount). It's probably much easier for you to pull the v2.0.7 changes in directly.

Even if you don't like my changes, you may want the first commit which only touches the instructions in BUILDING -- "rake gather" wasn't grabbing mkdio.h because you have to run generate.sh first to create it from mkdio.h.in.

Cheers
David Röthlisberger.