Bug 978454: Added code coverage report to test output.

Bug [978454](https://bugzilla.mozilla.org/show_bug.cgi?id=978454)

This patch adds support for code coverage reporting to the `test` command using `istanbul`:

![screen shot 2014-03-01 at 11 51 05](https://f.cloud.github.com/assets/41547/2301400/38ca9aea-a141-11e3-9921-f4afa634dd82.png)

It also generates a browsable HTML report, stored in `coverage/lcov-report/index.html`:

![screen shot 2014-03-01 at 12 25 15](https://f.cloud.github.com/assets/41547/2301401/38cd7a44-a141-11e3-9784-9bacd2cefa2a.png)

Note: this PR doesn't address any detected lack of code coverage in the current codebase.