Added caching support for compiled razor page assemblies.

Very useful for AppHarbor hosted apps. Saves the 10 to 60 second compile time every razor page takes on their CPU limited web server instances.

Only developed and tested against v3 (sorry, current project using this still on v3), but looks like it'll work verbatim for v4. And no unit tests yet, just wanted to throw it out there and see if this is in the right direction before putting time into that?