the get_equation JSON response should support URIs to the source content instead of embedding the source in the JSON response

[gcapiel]

For example, here we use sourceURI instead of source:

{ "createdAt" : "2014-12-10T23:49:16.247Z",
    "equation" : "5488dbf432d2e2e1286d580b",
    "format" : "png",
    "id" : "5488dbfc32d2e2e1286d580d",
    "sourceURI" : "http://somecdn.com/123456789",
    "updatedAt" : "2014-12-10T23:49:16.247Z"
  }