Enabled require("eslint") and exposed out CLI.

I've added the "main" key to the package.json, pointing it to lib/api.js. The api.js file exports /lib/eslint.js and /lib/cli.js. This enables those who have run `npm install eslint` to then require eslint in their project like so:

var eslint = require("eslint");

Prior to this change calling `require("eslint")` would throw an error.

In addition, developers can now use the CLI from their projects, without having to `require` it from the `node_modules` directory (which is generally not good). Developers can now use the CLI programatically, like so:

var eslint = require("eslint");
eslint.cli.execute(["-h"]);