Support turning off {{t}} helper's default <span>

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