TST: add coveralls automated coverage

Set up coveralls automated coverage report.  Based partly on
https://github.com/scipy/scipy/pulls/2907
TST: enable slow tests for travis
Merge pull request #297 from matthew-brett/add-coveralls

TST: add coveralls automated coverage

Set up coveralls automated coverage report. Based partly on 
https://github.com/scipy/scipy/pulls/2907