grunt based build system

Summary: First in the series of reviews of my new api explorer. Rest is coming up shortly. I have create task T1210 to keep reviews together. The first one introduces grunt based build system with npm packages to make it possible. npm install && grunt (having grunt-cli installed beforehand) should build the project and launch devappserver on 0.0.0.0:8080.

Test Plan:
grunt still produces expected result

running "grunt deploy" builds and deploys the app

Reviewers: alpert, joel

Reviewed By: alpert

CC: ben

Maniphest Tasks: T1210

Differential Revision: http://phabricator.khanacademy.org/D3005