Introduce Saber.QuerySet

QuerySet will be a chainable object, which a data source provides,
after processing. The API looks like:

    var source = new Saber.DataSource(data, schema);
    source.query(function (err, querySet) {
      /* Do something (filter, sort, aggregate etc) with the querySet
         In future, it can be like:
         querySet.where(...).sort(...).execute();
      */
      querySet.execute(function(err, models) {
         // I got the final models, use that data
      });
    });

TODO:

 - Remove Model object. Its doing absolutely nothing.
 - Add separate tests for fields, query-set

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Remove unused model.js and its tests

Model dosen't have any reason to be present in the source code. Now
there is no difference between the dev and dist tests which are run.

Signed-off-by: Rohan Jain <crodjer@gmail.com>