use ass for code coverage - issue #94

This PR adds code coverage reported at the console and via a dynamically generated HTML file at the time tests are run.  One may specify `NO_COVERAGE` in the environment to disable code coverage.  Notes:

* coverage instrumentation messes up line numbers.  If there are test errors, one should specify `NO_COVERAGE` to get true line numbers
* "executables" have shebangs removed, as they piss off the javascript parser
* all executables launched from tests now have tiny stub files to load dynamic instrumentation
* coverage reporting adds about 10% overhead
* I disabled coverage for code under `/client/` because
* 90.9% coverage?  you guys are awesome.

Reports look like this: http://people.mozilla.org/~lhilaiel/fxa-coverage.html