Make documentation

[mdpiper]

This PR adds user and API documentation, written with Sphinx, for the babelizer.

Todo:

• [x] Write a long-form example of wrapping bmi-example-c
• [x] Build the docs through Travis CI
• [x] Set up readthedocs site

[mcflugen]

@mdpiper I've added a docs build stage to the Travis CI that builds the docs and set up a readthedocs site.

[mcflugen]

@mdpiper I've added a few sections based on top-level docs (e.g. authors, changelog, etc.) and brought the contents of the readme file into the docs.

[mdpiper]

@mcflugen Thank you!

[mdpiper]

@mcflugen I'd like to make a long-form example of wrapping bmi-example-c. Should it go in the README (in Examples), or in a separate file?

It would make the README kinda long.

[mcflugen]

@mdpiper Probably in a separate document. The README is, as you say, already on the long side and is only intended to give a quick overview anyway.