Add an ArcanistJscsLinter.

Summary: Add a linter which wraps around JSCS.

Test Plan: Currently, we can't test this linter with out existing infrastructure. Specifically, jscs will only lint *.js files. I have done a fair bit of manual testing though.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9565