Don't override package data after it's been determined from
setup_package.py files
Use wheels for travis testing

Care is taken to use a numpy wheel whose version matches the scipy/astropy wheels.

Previous commit messages:
Use development astropy for test --coverage
Install coverage when needed
Add cython to the travis build environment, controlled by an environmental variable
Also added a few more words of advice to the travis part of the README
Restore -w option to build_sphinx
Change URL for numpy-specific wheels and make scipy an optional install
Fix latest stable numpy version
Several tweaks to improve performance and reduce unnecessary tests
Remove debugging statements and cython install in egg_only
Merge pull request #44 from mwcraig/travis-wheels

Use wheels on travis
Merge pull request #45 from mdboom/setup/package_data-dont-override

Don't override package data after it's been determined from setup_package.py files