BCL proxies

I've added proxies for BCL assemblies. Most classes that defined in JSIL libraries are just marked with JSStubOnly attribute, for some classes there are preciosity method-level JSExtrenal layout (we may improve this layout for all other classes later).
My next plan will be introducing new Test, that will use this proxies.
Note: as we have #504, we unable to translate System.Core before we fix it or apply #505 to hide problem.