Move source files to a src dir

This is best for a clean repository and for sanity while using
browserify
Add Gruntfile for browserify

New `Gruntfile.js` which builds `index.js` from `src/index.js`. Adds
`index.js` to gitignore.
Add grunt-contrib-clean

Now with a `grunt clean` task!
Add a build task

Use `grunt build` which is better the `grunt browserify`.
Add index.js to html file

Now the HTML document can use the built index.js
Add grunt-contrib-watch

Allows you to watch code changes and auto-build. Also uses livereload if
you have that browser extention.

https://github.com/gruntjs/grunt-contrib-watch#optionslivereload
Add a default task

Maps to 'build'