Use npm dependency instead of git submodule for QUnit

Picked the same version we already had (we used to point to 1.10.0pre, this is the released 1.10.0, every so slightly newer but compatible).

We should probably upgrade to a newer version at some point (v1.10.0 is from August 2012, latest v1.14.0 is from January 2014), but one step at a time :)