Generate JS classes from .idl files

So far, all of our element definitions are either empty or contain only reflection, so we can convert them to being generated via domenic/webidl-class-generator and depending on domenic/webidl-html-reflector and domenic/webidl-conversions. This generation will happen on prepublish (which also occurs when you do `npm install` or just `npm run prepublish`).
Add CustomHTMLHRElement (part of #33)
Use testem + browserify + es6ify instead of karma