[view/data-explorer][m]: (refs #14) introduce ability to change number of rows viewed.

* This also necessitated a nice refactor whereby DataExplorer subviews running off a common Backbone Collection (DocumentList) re-rendering themselves in response to changes in that Collection.

src/model.js

@@ -8,6 +8,10 @@ var my = {};
 // A Dataset model.
 my.Dataset = Backbone.Model.extend({
   __type__: 'Dataset',
+  initialize: function() {
+    this.currentDocuments = new my.DocumentList();
+  },
+
   getLength: function() { 
     return this.rowCount;
   },
@@ -19,13 +23,14 @@ my.Dataset = Backbone.Model.extend({
   // this does not fit very well with Backbone setup. Backbone really expects you to know the ids of objects your are fetching (which you do in classic RESTful ajax-y world). But this paradigm does not fill well with data set up we have here.
   // This also illustrates the limitations of separating the Dataset and the Backend
   getRows: function(numRows, start) {
+    var self = this;
     var dfd = $.Deferred();
     this.backend.getRows(this.id, numRows, start).then(function(rows) {
       var docs = _.map(rows, function(row) {
         return new my.Document(row);
       });
-      var docList = new my.DocumentList(docs);
-      dfd.resolve(docList);
+      self.currentDocuments.reset(docs);
+      dfd.resolve(self.currentDocuments);
     });
     return dfd.promise();
   }