First version of themeing!

Moved most of the asset/HTML to themes/default/ to follow on the issue
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
Moved prism to shared JS/CSS

The reason to do so is because Prism is used in admin as well as in the
post view.

Also, you cannot commit empty directory to git which caused error when
deploying ecrire (assets wouldn't precompile)
Whitespace!!!
Fixed sprockets' precompiler

Sprocket by default try to compile everything that resides in
app/assets. That meant that any images/font outside of app/assets
wouldn't be precompiled.

Because theme lives in themes/theme_name/, all assets in themes (besides
css|js) wouldn't be precompiled.