Clean up and prepare for custom layouts.

* Add some comments to the functions in Layout.hs to explain what they
do.
* Check if layout is found, and return an error if not (this would preferably be
returned as a 500 status code with a message).
* Move admin layout to admin folder, making the layout folder only for layouts.
* Add entry on layouts in readme, explaining how they work.
Merge branch 'master' of github.com:Tehnix/HsCMS