add flake8 & docs build check to travis

[mastersplinter]

Updated:
This PR:

• adds flake8 to travis.yml
• makes flake8 only test pandera/pandera.py
• make travis.yml use requirements.txt
• fixes a couple of minor flake8 compliance issues
• adds a final step to travis.yml which will error if the docs produce either an error or a warning. This is useful for two reasons:
  • it avoids introducing errors in documentation
  • autodoc/sphinx will warn about malformed docstrings in a way that flake8 doesn't always catch

to address #54 #49 #47

[mastersplinter]

The current test failures are correct (Travis is failing correctly).

A large number of these issues have changed in #50, so suggest re-running this after that PR merges.

[cosmicBboy]

we should fix all the lint errors resulting from adding flake8 to CI.

should probably ignore the F401 errors in the init file

[mastersplinter]

@cosmicBboy This PR is now ready for re-review.

Changes are:

• changes flake8 to only apply to tests/ and pandera/pandera.py, not pandera's init.py.
• adds a final step to travis.yml which will error if the docs produce either an error or a warning.
• fixes errors so that the new flake8 and docs build checks pass successfully.