{{t}} helper uses registerBoundHelper

 * use `registerBoundHelper` to register `{{t}}`
 * `{{t}}` no longer supports passing a `tagName`
 * drop support for `I18N_TRANSLATE_HELPER_SPAN` and remove corresponding `taglessTranslateHelperSpec.js`
 * tests use `fooBinding="bar"` instead of `foo=bar`, per behavior of `registerBoundHelper`
 * mark compatibility with all Ember 1.x
 * remove vestigial `warn`, `isBinding`, `uniqueElementId`, and `TranslationView`

Resolves #131, #66 
I have a local branch where I started tinkering with putting canary back in - you probably have one as well, but if it's useful I can push it up. It's really just a selective revert of 58f7b0d, plus adding Handlebars 2.0 to the vendor dependencies. Not all green yet :red_circle: