codecov, deploy git tags to pypi with travis

[wholmgren]

Two changes here:

1. Adds support for the codecov tool. This seems to be the recommended test coverage tool these days. I'll keep coveralls for now.
2. Travis will deploy tagged versions to pypi. For example, the release process should be as simple as: git tag v0.6.0-rc1; git push --tags

[wholmgren]

Guessing this will need some iteration on master, so merging now and we can revert if necessary...

[wholmgren]

Wow. It actually worked.

$ pip install pvlib
Collecting pvlib
  Using cached https://files.pythonhosted.org/packages/f9/99/629c78634fdb3857622ad8c2556815e8246f92289efe3ca897fb34016f25/pvlib-0.5.2-py2.py3-none-any.whl
^C
$ pip install pvlib --pre
Collecting pvlib
  Downloading https://files.pythonhosted.org/packages/e2/b2/63eb6a44e593f5f586dc49fb769e0d5edd7b70a9113cc21974ffb679802a/pvlib-0.6.0a2-py3-none-any.whl (17.1MB)
    100% |████████████████████████████████| 17.1MB 2.6MB/s
Requirement already satisfied: six in ./miniconda3/envs/pvlibpypitest/lib/python3.6/site-packages (from pvlib) (1.11.0)
Collecting pandas>=0.14.0 (from pvlib)
  Using cached https://files.pythonhosted.org/packages/78/78/50ef81a903eccc4e90e278a143c9a0530f05199f6221d2e1b21025852982/pandas-0.23.4-cp36-cp36m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl
Requirement already satisfied: numpy>=1.10.1 in ./miniconda3/envs/pvlibpypitest/lib/python3.6/site-packages (from pvlib) (1.15.1)
Requirement already satisfied: pytz in ./miniconda3/envs/pvlibpypitest/lib/python3.6/site-packages (from pvlib) (2018.5)
Requirement already satisfied: python-dateutil>=2.5.0 in ./miniconda3/envs/pvlibpypitest/lib/python3.6/site-packages (from pandas>=0.14.0->pvlib) (2.7.3)
Installing collected packages: pandas, pvlib
Successfully installed pandas-0.23.4 pvlib-0.6.0a2