enable both relative and absolute treePaths (npm v3 fix)

related PR to ember-cli-ic-ajax rwjblue/ember-cli-ic-ajax#9

lib/models/addon.js

@@ -307,7 +307,7 @@ Addon.prototype.treeFor = function treeFor(name) {
   @return {tree}
 */
 Addon.prototype._treeFor = function _treeFor(name) {
-  var treePath = path.join(this.root, this.treePaths[name]);
+  var treePath = path.resolve(this.root, this.treePaths[name]);
   var treeForMethod = this.treeForMethods[name];
   var tree;