Add code coverage metrics to unit test suite

[shawnbot]

Per #1673, we are going to add code coverage metrics as part of our 1.0 release so that we can demonstrate (hopefully) improvement in future releases. To do this, we need to:

• Ask our friends in Engineering (and Infrastructure) which code coverage tool to use

  • James Seppi mentioned that our Before You Ship guide requires Code Climate for static analysis, but I need to follow up on whether that applies to us
  • Noah Kunin asked that we use Code Climate and said that it "seems to be an emerging standard"

  In summation: we'll use Code Climate.

• Add that tool to our project

• Integrate code coverage into our CI setup

• Maybe: document what code coverage means on the site

[shawnbot]

This is basically done in #1702. Let's create a new issue to document what code coverage means, and how we deal with it in pull requests and/or releases.

[shawnbot]

This is done, and I've moved the documentation tasks over to #1704.