Retrieve profile from url

[larrysalibra]

Blockstack Auth generally only returns a url to the profile in the profile_url field.

This change was made to avoid exceeding server max request sizes.

The relevant code in blockstack.js is here: https://github.com/blockstack/blockstack.js/blob/e8b0c2afaaa432283b47726a89b6388699b38125/src/auth/authApp.js#L205

We should update this library to have similar behavior. Until this happens, the profile will typically be nil.