Working on an express.js version of jslint to support aliasing, JSON API etc

Hello! Thought I'd pitch in with a nodejs version to fix issue #17, allowing API access, aliases, and the other cool things enabled by having a server.

This isn't supposed to be merged yet by the way :) Just to discuss.

- It's up on [Heroku](http://lint-explain.herokuapp.com).
- At mo I've got to parity with the current site, and added [JSON](http://lint-explain.herokuapp.com/wrap-an-immediate-function-invocation-in-parentheses.json):
- Just add .json to a lint error URL to get the same content as JSON: e.g http://lint-explain.herokuapp.com/wrap-an-immediate-function-invocation-in-parentheses.json
- To run, clone and `npm install && node src/index.js`.
- To run tests `npm test` (not many yet).

Let me know what you think!