Use QueryResult Object for returning data from backends

[rufuspollock]

At present do not have a QueryResult but just return a set of "document" dicts/hashes (and have the Backend explicitly set total on Dataset in form of docCount. Issues:

• Will need this once we have facets
• Every backend needs to repeat setting of docCount on backend

Pros of current system:

• We allow Backends to whatever they want without having to conform to a specific QueryResult construct (and anyway Dataset is sort of our QueryResult ...)

Structure of query result:

{
  total: // total number of results
  hits: [ // as per elasticsearch
    {
       _score:   // optional match score
       _type: // optional document type
       _source: { // document data
       }
    } 
  ],
  facets: { //
  }
}