CI: add in support for coverage via Codecov
so there is a nice browser extension to show the coverage on github: https://github.com/codecov/browser-extension#codecov-extension

will have active coverage once this PR builds. I think I have it set for PR's to show coverage updates (where the travis/ci line is).

cc @jorisvandenbossche @shoyer @TomAugspurger @sinhrks @wesm 
 you can look at/modify things here: https://codecov.io/github/pydata/pandas/commits 
