nimrod: rewrite formula.

The new formula installs nimrod completely from source, following
the instructions from the readme.md in Araq/Nimrod. This method
does not rely on having a proper bootstrap stage supplied by
upstream, so theoretically this allows nimrod to be compiled
on any Homebrew setup.

The new formula also installs the 0.9.6 version of nimrod, and
attempts to stay as close to the upstream source installation
instructions as possible.
nimble: Add formula (nimrod's package manager).
nimrod: add 0.9.6 bottle.