Adds karma.conf.js file to support the Karma testrunner.
Add ability to derive attributes from multiple source models.

Documentation to follow.
Documents support for multiple source models.
Merge branch 'master' into pr/3

Conflicts:
	src/view-model.js
source_models shouldn't be set on the model itself, you need to use the
accessor.  minor refactoring and minor documentation change.

* Access to the source_model or source_models should be very similar,
both through this.get("source_model") or this.get("source_models").foo.
Setting the source_models directly on the viewmodel instance might have
unintented consequences (violating least suprise), ie naming conflicts.

* Changing readme documentation to match the source_model example.
Trying to get the karma build to be green (with the new requirejs
changes), but failing at the moment.

* TODO: if someone can make this pass, that'd be amazing.