Thread-safe and asynchronous core (part 3)

Part three of the big refactor. This commit includes:
+ Abstraction of platform-specific data and operations (IPlatformHelper)
that are used by Engine.
+ Refactor of the loading layer using interfaces (ICartridgeLoader).
+ Cleaner architecture of the core: model classes are in root, engine
classes in the Engines namespace. (Updated the WP7 and main Core
projects)
+ Added WherigoHelper as an entry point for client use-cases.
+ Added UI feeback from Engine about the business of the Lua execution
queue and the UI dispatch pump. (See bindable property Engine.IsBusy).
+ Made Cartridge extend Table.
+ Documented parts of the code.

Known issues / Todo:
- Write standard IPlatformHelper implementations for other projects:
iOS, Android.
- Write standard init-and-start methods in WherigoHelper (sync and
async).
- Potential concurrency problems in Cartridge. (Write thread-safety.)
- Thourough documentation.