Fix getText method to deal with nestes nodes, e.g. span inside header

I didn't find anything to update the minified file in `dist`, so for now I changed the tests to use the src file. The updated `getText` implementation used to be in QUnit, for example [it was in 1.0.0](http://code.jquery.com/qunit/qunit-1.0.0.js). Without the patch, `generateId` fails since the `text` argument is undefined.