Allow Bower to manage QUnit.

This PR moves QUnit from /tests/vendor to /vendor and sets up `bower.json` so that we can use Bower to manage the QUnit dependency in the future.  Bower is not required to run the tests, but it can be used if desired.

Future updates to QUnit (or any of the other dependencies) can be done with the following commands:

```
# update bower.json to use the new version
bower install

# since /vendor is ignored this will only add the files that have
# explicitly added to git
git add vendor

git commit
```