Add a JSON linter.

Summary: Provide bindings for JSONLint, which is a useful tool for linting and validating JSON. Theoretically, this could be done with pure PHP, however it would not be trivial (json_decode, for example, does not provide any context as to JSON validation errors).

Test Plan: Wrote and executed unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8988