Provide mscorlib as npm package

[kskalski]

Looks like the only way to include mscorlib.ts into a project is to download it from github and then add it as part of my project's source code. I would rather have it as dependency, e.g. add a versioned package to npm's package.json and make it land in node_modules/netjs/

This might have some implications on how it's referenced from generated code though