Add automated windows CI using appveyor

This PR adds automatic testing on windows using appveyor.com. 

It is based on, and incorporates parts of, https://github.com/rmcgibbo/python-appveyor-conda-example by @rmcgibbo, which is in turn a fork of https://github.com/ogrisel/python-appveyor-demo by @ogrisel. Honestly, they did all of the hard work!

To see an example of the report produced, go to: https://ci.appveyor.com/project/mwcraig/astropy/build/1.0.8

It does not test the same range of python/numpy variants as the travis tests, but does cover the basics: python 2.7 and 3.4 on both 32 and 64 bit.

I took a quick look at the failures in 64-bit...they look like real test failures rather than problems with the appveyor configuration.

Before this is merged we would need to:

- [x] setup up an astropy account on appveyor (free for open source projects)
- [x] link the astropy repo to the appveyor account (done from appveyor, not github)

I hope to soon make a couple of additional pull request mining the goodness in the appveyor examples to automatically build conda packages of the dev version of astropy (for travis) and for affiliated packages (for linux and windows)....but this was the most straightforward place to start.