Making this usable as a library

Hiya; I'm just doing this pull request to start a conversation or notify you or something – the changes I've made break your index.html, and I haven't fixed that, so it shouldn't be pulled directly.

Anyway, I changed it to make it usable as a library, so I could use these as default avatars in another project (https://github.com/mozilla/towtruck).  This basically removes touching any concrete elements in the page (it just uses one canvas element that isn't added to the document).  It also fixes some jshint stuff, makes the size configurable, and removes/changes randomBetween().

I've set it up to use require.js optionally, though it should still work fine without it as well.