Added support to .pyi stubs.

Pull Request for  #345 that now is in "priority"

I had to make some decisions, like that \_\_init\_\_.py don't change the extension, shouldn't be a stub, and python use it to know the modules.

Also, to allow import scripts, the script sometimes look just .pyi files, and other times look .py and .pyi (first .py).

If don't if that's the solution you want.