Fix bugs, add moment-shim and improve test

@johnotander this is an general improvement to your addon, which was already good =)

- Register all helpers with Ember.Handlebars.helpers to fix a "undefined bug". It´s is happening in ember-cli 0.1.x
- Add ember-cli-moment-shim to avoid global moment (https://github.com/jasonmit/ember-cli-moment-shim)
- Improve tests to work with Date, Moment and literal dates
- Create an index.hbs to show off all helpers

I´ve based my code on https://github.com/stefanpenner/ember-moment, maybe it´s good to reference this project on README.md