Will/diff cover integration

This integrates the `diff-cover` tool into Jenkins, while simplifying the existing coverage rake commands.  The basic idea of diff coverage is simple: for each line you add/modify in a PR, `diff-cover` will tell you whether that line is covered by the test suite.

The `diff-cover` tool exists in a separate edx repo: https://github.com/edx/diff-cover.  The README in the repo explains how the tool works.

Changes in this PR:

* Consolidates rake tasks `coverage:xml` and `coverage:html` into a single command `rake coverage`.

* Cleans existing coverage reports between each test run.

* Adds a user-friendly error message when `rake coverage` is run without first running the test suite to generate coverage info.

* Generates HTML and console diff coverage reports using `diff-cover` tool.

I want to get this merged soon so we can start collecting diff coverage info for existing pull requests.  This will ensure that (a) the process is working smoothly before we integrate it into code reviews, and (b) we have base-line numbers to help us determine reasonable requirements for coverage.

Suggested reviewers: @jzoldak and @dianakhuang