title: What are the available condition constants under uiGridConstants.filter?
body: <p>Did I miss the link that documents ALL the available uiGridConstants for uiGrid fields in one place?  I was working on columnDefs filter and found out EQUAL does not work.  That's when I realized there is no central documentation for all the condition constants for uiGridConstants.  </p>
  
  <pre><code>columnDefs:
   [ {   }
      , { field: '_pointXID', filter
          : {
              condition: uiGridConstants.filter.CONTAINS, placeholder: '%PATTERN%'
          }, headerCellClass: $scope.whatfilter}
      , { field: '_statU16', width: "5%", filter
          : {
              condition: uiGridConstants.filter.EQUAL, placeholder: '=='
          }, headerCellClass: $scope.whatfilter}
      , { field: '_valDoub', width: "5%", enableFiltering: false }
  
   ]
  </code></pre>
  
  <p>EQUAL's not one of them, and heck how do I know?!</p>