title: Adding filters with "And" and "or" in 2.0rc1
body: <p>I am having problem in using filters with "And" and "or" clause. Basically i want to get any userstories having text "Mobile" or "Testing"</p>
  
  <p>I am trying like this : </p>
  
  <pre><code>                    _createFilters:function(){
                      var filter = Ext.create('Rally.data.QueryFilter',
                      {
                          property: 'Name',
                          operator: 'contains',
                          value: 'Mobile'
                      }
                  );
  
                      filter = filter.or(Ext.create('Rally.data.QueryFilter',
                      {
                          property: 'Name',
                          operator: 'contains',
                          value: 'Testing'
                      }));
  
                      return filter.toString();
  
                  },
                  _makeDefectsStore:function(ref){
                      Ext.create('Rally.data.WsapiDataStore',{  // will fetch userstories that have defects
                          model: 'User Story',
                          limit: "Infinity",
                          context: {
                              project :'/project/xxxx',
                              projectScopeUp: false,
                              projectScopeDown: true
                          },
                          autoLoad: true,
                          fetch:['FormattedID','Name','Defects','Feature'],
                         filters: [this._createFilters()],
                         scope:this,
                          listeners: {
                              load: this._onAllDefectsLoaded,
                              scope: this
                          }
                      });
                  }
  </code></pre>
  
  <p>Here filter is returning null, otherwise all the userstories are returned without filtering . Please provide suggestion/fix</p>