Set up coveralls about astroplan HOT 1 CLOSED

@ejeschke @bmorris3 @jberlanga We now have Coveralls activated for the astroplan repo.

The way this works is that travis-ci runs python setup.py test --coverage and then posts the html coverage report to Coveralls:

https://travis-ci.org/astroplanners/astroplan/jobs/60377593#L492
https://coveralls.io/builds/2479267

The coverage in % is mentioned on a badge in the README and you can click on it to see the latest coverage report:
https://github.com/astroplanners/astroplan

Coveralls also comments on pull requests (PRs), with a link to the coverage report for that PR.
Now this can be a bit annoying, but I'd like us to give it at least a try and if we don't like it we can turn that off.

Now all that line coverage does is show which lines are executed at least once by the tests. I.e. it's a quick way to find if / else clauses or whole functions or files that are completely untested. Not more, not less.

from astroplan.