I've made requireStylesheet also check the loadedThemes for the
requiredStylesheet.
In the event that we have 2 stylesheets with the same name `editor.css`
and `editor.css` I want the `static/` directory stylesheet to be above
the userTheme stylesheet, so that proper cascading occurs. This commit
also adding class names to the styles.
These changes are mostly aesthetic in nature. I just thought it would
be a good idea to have theme loading be parallel to package loading as
much as possible. So I localized more logic on the `atom` global.
We'll look first in the user themes directory, then in the built-in
themes directory. This allows us to default to 'IR_Black' and not crash
if the user doesn't setup their `~/.atom/themes` directory.
