Adds grunt task for generating docs

Uses `docco` to generate documentation, also used by `underscore` and `backbone`.

    $ grunt docs             # Generate documentation
    $ open docs/index.html   # View in your favorite browser

`docco` uses your code's comments to annotate the source code. It's very simple -- comments are on the left, and code is on the right.

To annotate your source code, use single-line comments (`//`) above the code:

    // Adds two numbers and returns the result.
    function add(x, y) {
        return x + y;
    }