Add task to Gruntfile to build parser

Summary:
As it is it's not too difficult to just run `node
src/parser-generator.js`, but I thought it would be nice to have both
build tasks in the same file just so it would be easier for anyone else
to figure out how to build the parser.

Also added some quick instructions on how to build it in the README.

Test Plan:
  * Make some change to the parser generator
  * Run `grunt parser`
  * See your changes have been compiled into parser.js

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D6046