Improve response for channels

[Tmw]

As discussed with @jchn we should prettify the response from our API for the GET /pins. It should include a selected default channel_id as well as an array with the channels available under the account. Example:

{
  "default_channel_id" : "CXYZAABCDIEO",
  "channels" : [
    {"name": "announcements", "channel_id": "CAIUEMDU"},
    {"name": "general", "channel_id": "CXYZAABCDIEO"},
    {"name": "gifs", "channel_id": "CAIUEMDR"},
    {"name": "bootcamp", "channel_id": "CAIUEMDL"},
    {"name": "development", "channel_id": "CAIUEMDP"}
  ]
}

Any thoughts, @jchn?