fixed issue with overriding core components

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "nvd3",
-  "version": "1.1.13-beta",
+  "version": "1.1.14-beta",
   "homepage": "http://www.nvd3.org",
   "authors": [
     "Bob Monteverde",

src/core.js

@@ -2,14 +2,14 @@
 var nv = window.nv || {};
 
 
-nv.version = '1.1.13b';
+nv.version = '1.1.14b';
 nv.dev = true //set false when in production
 
 window.nv = nv;
 
-nv.tooltip = {}; // For the tooltip system
+nv.tooltip = nv.tooltip || {}; // For the tooltip system
 nv.utils = nv.utils || {}; // Utility subsystem
-nv.models = {}; //stores all the possible models/components
+nv.models = nv.models || {}; //stores all the possible models/components
 nv.charts = {}; //stores all the ready to use charts
 nv.graphs = []; //stores all the graphs currently on the page
 nv.logs = {}; //stores some statistics and potential error messages