Support for Ellipsis and ...

Added Python 3 specific support for ellipsis. See test cases for examples.

I have some doubt about the way I introduced the `ellipsis` class, which is the type of `...` and `Ellipsis`. I've added it as a class to `builtins.py`, however, that makes code like `a = ellipsis()` legal, which it is not. I don't know how to have `ellipsis` as a class, while at the same time hiding it.

Fixes #524.
Add comment mentioning that `ellipsis` isn't actually defined in Python builtins (only `Ellipsis` is), so strictly speaking we shouldn't include it in builtins. However, there are other cases like this (e.g., `module`) and all similar issues can be fixed separately, so this is fine for now.