Python 3 support for numpy.

As I stated in #96, Python 3 support is missing in most formulas. Here it's my support of Python 3 for numpy. Also due to homebrew itself has deprecated `depends_on 'nose' => :python` and is in favour of using resources like e.g. ansible.rb does. (See Homebrew/homebrew#27326) I choose using new way to handle `nose` package. 

In my test, it works fine. As a beginner new to ruby, I only submit the pull request for numpy.rb. If this pulll request is accepted, I will adding Python 3 support for other formulas.