Only use contextual components

[Robdel12]

For emberx-select 3.0 we would like to only support contextual components. For example:

{{#x-select value=model.status as |xs|}}
  {{#xs.option value=1}}Active{{/xs.option}}
  {{#xs.option value=2}}Inactive{{/xs.option}}
{{/x-select}}

This allows us to remove registering code and possibly provide speed improvements. This means emberx-select will only work with Ember 2.3 or above.