Remove lang attribute.

Dojo's parser now looks for the lang attribute on ancestor elements and
sets widgets' `lang` property to that string. `dojo/i18n` works on
`navigator.language` and when the lang attribute and
`navigator.language` do not match, localization lookup for widgets will fail.

Fixes #4
Add comments detailing when `app/main.init()` executes, how it should be used, and where to place content in index.html