Implement web platform (JSIL) stubs and update Protobuild

This provides initial support for a Web platform driven by JSIL.  Currently all platform-specific code are empty stubs (so the web platform won't actually work at the moment), but it does set up the requirements such that the code will compile.

It also updates Protobuild, which includes support for the Web platform by the way of invoking the JSIL compiler after an executable project is built.  This allows for transparent compilation to Javascript without developers having to explicitly configure or setup anything.