Explain how to define custom test helpers

This update proposes a way to use Ember's built in helper registration.

I apologise if I've poorly grasped any of the concepts around modularization / ember-cli, but hopefully this is a good first stab at it.  I've tested the 'style' I've proposed and it does work, but if there's a more idiomatic way, I'd love to hear :)
Doh!  Intermediate variable not needed!
Add notes about .jshintrc
Merge pull request #965 from mfeckie/patch-2

Explain how to define custom test helpers