#579 Support for metricless queries

[itayw]

Resolves #579

Added support for metricless queries, tests document process.
Example:

it("should perform an last_n_items query w/o metrics", function (done) {
  var query = {
    timeframe: 'last_1_items',
    interval: 'minute',
    dimensions: ['attribute'],
    metrics: [

    ],
    collection: this.collection
  };
  joola_proxy.query.fetch(this.context, query, function (err, result) {
    if (err)
      return done(err);

    expect(result).to.be.ok;
    expect(result.documents).to.be.ok;
    expect(result.documents.length).to.be.greaterThan(0);
    expect(result.documents[0].values.attribute).to.equal('test');
    return done();
  });
});

Related to:
joola/joola.datastore-influxdb#4
joola/joola.datastore-mongodb#10