Enable some basic docstring testing; possibility to extend testing later

[pep257](https://github.com/GreenSteam/pep257) seems like a nifty little library that checks compliancy with PEP257. We don't follow everything from PEP257 (it has differences with NumPy docstrings), but some checks are still useful. It should be relatively straightforward to extend to some more NumPy specific checks if we want to do that in the future.

Right now this makes Travis check for very basic things like having a one-line summary that ends with a period, closing the docstring on a separate line, and not having blank lines before or after the docstring.