Allow installing into an empty virtualenv

[ecederstrand]

This moves the import of numpy into a custom build_ext command so we can:

• postpone the import of numpy to a time where setuptools has had a chance to install it
• record the build dependency in setup_requires so it's installed and and removed again automatically

Fixes #2

[ecederstrand]

@iseekwonderful Ping?

[ecederstrand]

No longer using this package. Closing.