UI Plugin API: Implement 'apiCall' helper

Adds a helper method to standardize how plugin writers handle server calls,
without having to directly invoke jQuery.ajax. It will correctly sanitize data
and ensure all required parameters (e.g., session key data) are passed.

ui/scripts/plugins.js

@@ -12,6 +12,15 @@
   };
 
   var pluginAPI = {
+    apiCall: function(command, args) {
+      $.ajax({
+        url: createURL(command),
+        success: args.success,
+        error: function(json) {
+          args.error(parseXMLHttpResponse(json));
+        }
+      })
+    },
     addSection: function(section) {
       cloudStack.sections[section.id] = $.extend(section, {
         customIcon: 'plugins/' + section.id + '/icon.png'