Add contributing/style guide

[heartsucker]

Even just to keep myself organized. Off the top of my head, it should include things like what to log and at what level to log it.

[lucab]

Some mention of rustfmt, perhaps? It would be nice to know that I can just blindly cargo fmt all PRs, or that there is a custom rustfmt.toml config manifest in case of customizations.

[heartsucker]

I'm using rustfmt = 0.9.4 right now, but I haven't set up a rustfmt.toml. If you have have recommendations, I'm all ears. Annoyingly, the RSA prime in src/rsa/mod.rs doesn't respond well to the annotations telling rustfmt to ignore it, so I have to do a patch and then checkout that bit each time grumble grumble.

[heartsucker]

And if that was all tricked out and added, it would actually be nice to enforce cargo fmt in the Travis build to make future diffs cleaner.