Windows CI using AppVeyor - also change to a MinGW binary

How to get started:
Sign up at appveyor.com for a free open source plan
Probably have to authenticate to let AppVeyor see your GitHub repos, I forget
Click New Project, find this repo in the list and click Add
Push a test commit, or manually go to projects, HDF5.jl, click New Build

Also recommended so Travis and AppVeyor coexist nicely on pull request status indicators:
Follow instructions here http://github-multi-status.herokuapp.com/ to add an extra webhook for Status events