Simplified application layout.

The themeing is now looking good. One of the area that could have been
confusing was the layout (application.html.erb). Most of the
confusion was taken away by using high level functions that wraps the
head.

Conditionals have also been removed to avoid screwing up the layout on
assumptions.

Added shared assets to share between admin and normal. This allow stuff
like admin menu to keep the same look and feel while having DRYed CSS
Modified JS structure to reflect CSS

Because JS and CSS should follow a similar structure to make sure it's
easy to understand for people wanting to build a theme