Consolidated coverage report generation into one rake command.
Added diff coverage reports using diff-coverage tool.
Clean the report directories before each test run,
to ensure we don't get coverage results from a
previous test run.
Added diff-cover as a requirement (installed from github)
Fixed bug in tests.rake in which rm would be called with no args.
Updated to current tag of diff-cover
Merge branch 'master' into will/diff-cover-integration
Added diff coverage printout to rake coverage command
Added warning message when no coverage information found.
Added comments to tests.rake
Merge pull request #68 from edx/will/diff-cover-integration

Will/diff cover integration