Add categories to Cargo.toml

[shepmaster]

Hi! crates.io now supports categories, which are a curated list
of topics aimed at helping an end-user coming to crates.io looking for
"a crate to do ______".

We're sending pull requests to selected crates to add categories in order to help
populate the categories and seed their usefulness. We've made a guess at the best
category/categories for this crate; if it doesn't fit, please feel free to take
a look at all the available categories and their descriptions and
the slug values that should be specified in your Cargo.toml and pick
different ones. If you have a category in mind that isn't available, you can
send a PR to this file on crates.io to propose additional
categories.

Crates can have up to 5 categories, and uploading categories to crates.io
currently requires publishing a new version with a cargo nightly from 2017-01-18
or later (it needs to contain this PR).

We've published a blog post with further details about categories.
The blog post also talks about the new crates.io support for CI badges, which
you may be interested in adding as well.

Please let me know if you have any questions!