Jasmine tests for verification / payment; small bugfixes

* Add Jasmine tests for a good chunk of the payment/verification JavaScript.
* Found and fixed a bug: the payment button should be enabled if there are no suggested prices (and no radio buttons).
* Some general cleanup / refactoring.

Reviewers: @rlucioni @AlasdairSwan 
You might want to add a `trackLink` property to these spy objects. It's another Segment method we've started using.