Add docstring checker

[cdeil]

It would be great if there was an automatic docstring checker for Astropy and affiliated packages available.

See astropy/ccdproc#338 as an example.

There's also examples in the Astropy core where a missing whitespace before a colon leads to the docstring not being rendered correctly:
http://docs.astropy.org/en/latest/api/astropy.io.ascii.Ipac.html#astropy.io.ascii.Ipac.write

While I was writing this issue I remembered that I had thought of this before and found this tool which basically does it:
https://github.com/PyCQA/pydocstyle

What remains to be done is to make it work with our docstring format
PyCQA/pydocstyle#129
and then to apply it to Astropy core and let affilated package authors know about it.

cc @bsipocz

[bsipocz]

A related issue in core astropy/astropy#5504

[pllim]

We have RTD run now in PR that can turn warning into error.