Add some lints and reformat some code

[killercup]

Along the lines of this article, this

• enables more lints as warnings (especially missing-docs) and fixes some of the trivial things
• adds clippy lints (optional), and fixes warnings
• reformats the files with rustfmt (I chose not to include every change, e.g. for long lines or arrays)

This also adds a basic config for TravisCI. I you want to use it, you can easily generate and add a secret Github token to upload docs, and enable test coverage collection by re-enabling some of the commented lines.

Feel free to pick and choose which commits you want to merge.

[buntine]

Oh, thanks so much. I really should have read your article! I will fix the remaining warnings over the next day or so.

[killercup]

Cool! The only remaining warning should be missing-docs. When you've fixed those, you could switch from #[warn(…)] to #[deny(…)] to turn them into hard errors.