This is a patch to update the documentation on the typing module. It expands the section on type aliases and adds a section on [NewType][0].

Since support for NewType was [recently added][1] to mypy, it seemed like a prudent time to update the docs to describe this new feature.

The section on type aliases was mainly expanded because the distinction between type aliases and NewType could be potentially confusing, so adding extra clarification seemed like a good idea.

  [0]: https://www.python.org/dev/peps/pep-0484/#newtype-helper-function
  [1]: https://github.com/python/mypy/pull/1939