Added coverage for test suite.

This PR provides a configuration file, and a Makefile target, to use the `coverage` package to report how well the unit tests cover the source code. If you say `make coverage`, it will run the unittests as with `python -m unittest discover`, and generate an html report, in `coverage_report/index.html`.

This assumes that the `coverage` package is available. The parallel tests are not included in this coverage measurement.

The configuration excludes the coverage of the test cases themselves, and things outside distarray. (E.g. numpy.)