Split stylesheet.php entry point in several files

Summary:

• classes are now in app/Assets/
• entrypoint calls Laravel autoloader, then the run method call

This is a follow-up for f259a9c16ac1.

Test Plan: Check /stylesheet.php?resource=less&name=app

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D418