Added JSStubOnly attribute for simplification creation of class sutbs.

I've worked on compiling full mscorlib and System.Core using JSIL.
Part of classes are implemented in current JSIL js libraries. I want JSIL to create only stub for such classes. Currently it is not possible - if we mark class with JSExternal, JSIL doesn't generate method stubs for such class.
I've introduced JSStubOnly attribute. If class is marked with this attribute, only stubs for all method inside this class should be generated. This attribute give me possibility to easily create custom Proxy assembly, that mark all classes implemented in js libraries as JSStubOnly. So I may use full framework with just simple proxies like:

```
    [JSProxy(typeof(Console), JSProxyMemberPolicy.ReplaceNone, JSProxyAttributePolicy.ReplaceDeclared, JSProxyInterfacePolicy.ReplaceNone, false)]
[JSStubOnly]
    public static class System_Console
    {
    }

    [JSProxy(typeof(Debug), JSProxyMemberPolicy.ReplaceNone, JSProxyAttributePolicy.ReplaceDeclared, JSProxyInterfacePolicy.ReplaceNone, false)]
    [JSStubOnly]
    public static class System_Diagnostics_Debug
    {
    }
```