Closing issues from pull request body text

[Jeffrey Warren]

Just a heads up to gsoc students -- if you use the phrase "closes #575" or another issue # in your pull request body text, the corresponding issues will be closed once the PR is merged. 

This is really helpful for not leaving open issues where the problem was resolved, and I recommend it!

https://github.com/blog/1506-closing-issues-via-pull-requests