Show lead's info.

* Add a resource to show a specific lead. Since the list of leads is still
remain present on the page when shows a lead, this resource should be nested
under the leads resource.
* Create a route object to pull down the specific lead.
* In the template, the single quote (') leaves a trailing whitespace after the line.
* The outlet tag placed at the end of the template contains the lead's info, since
the lead route is nested under leads.
Let user edits status and notes in place.

* Create an array of statuses.
* The 'click' attribute in the form specifies a function to call when user click the submit
button. Ember expects the controller to define this function, that must be within 'actions'
since it's being called from a template action.
* The controller should be an ObjectController because it wraps a single lead model.
* save() will actually send an AJAX PUT request to our Rails API, with 'api/v1/leads/<id>'.
Prompt the user to save changes before switching to a different lead.

* More info: https://github.com/vicramon/ember-tutorial/blob/master/chapters/context-did-change.md