Reset warnings and sys modules completely before and after logging tests to avoid side effects.
Fix API links in modeling docs
Fix module link in coordinates
Merge pull request #1750 from astrofrog/fix-logging-tests-side-effects

TestComparisonUfuncs.test_comparison_valid_units failing with Numpy 1.8.0 and Numpy dev
Merge pull request #1753 from astrofrog/fix-api-links

Fix API links