Add github-linguist-grammars gem

The purpose of this gem is to package up the language grammars that are
used for syntax highlighting on github.com. The grammars are TextMate,
Sublime Text, or Atom language grammars, converted to JSON and given the
filename SCOPE.json, where SCOPE is the language scope that the grammar
defines.

The github-linguist-grammars gem packages up all the grammars, and also
exports a Linguist::Grammars.path method to locate the directory
containing the grammars.

To build the gem, simply run `rake build_grammars_gem`. The grammars.yml
file lists all the repositories we download grammars from, as well as
which scopes are defined by each repository. The
script/download-grammars script takes that list and downloads and
processes the grammars into the format expected by the gem.