Support turning off <span>s by default

If `Ember.FEATURES.I18N_TRANSLATE_HELPER_SPAN` is set to `false`, the `{{t}}` helper will not emit a `<span>` by default.

If `Ember.FEATURES.I18N_TRANSLATE_HELPER_SPAN` is set to `true`, the `{{t}}` helper will use a `<span>`.

Otherwise, the `{{t}}` helper will use a `<span>` AND emit a deprecation warning.

In all cases, the `{{t}}` helper now uses Metamorph tags. If a `tagName` is specified, the helper gives that element a unique ID based on `Ember.uuid`.

Resolves #33 and #111