Fix type errors

As a follow-up to the last pull request, I've gone ahead and fixed the type errors in the non-contrib modules.

I think this resolves most of the issues, although there are a few things that I didn't touch that you may want to take a look at:
* I wasn't sure what the purpose of `ReadableStream` and `WritableStream` were.  I couldn't find any references in the node documentation.  So I mostly left them alone.
* `stream.Duplex` currently doesn't inherit from `stream.Writable`.  This needs to be fixed, but due to [Issue 250](https://code.google.com/p/closure-compiler/issues/detail?id=250) and lack of support for mixins, this won't be easy.
* Some of the changes to the `stream` module may break code using really old versions of node.  This namespace has changed a lot and I'm not very familiar with the old interfaces.  Although it looks safe, I'm sure there are some corner cases.
* I'm not sure why many of the module namespaces are annotated as dictionary objects.  So I left them as-is.

Cheers,
Kevin