Make GH branch name customizable

PolymerElements/issues/101

iron-component-page.html

@@ -291,6 +291,15 @@ <h2><span>[[active]]</span> <span class="version" hidden$="[[!version]]">[[versi
             observer: '_srcChanged',
           },
 
+          /**
+           * The name of a brnach used to fetch source for gh-pages.
+           * If not specified `master` is used.
+           */
+          branchName: {
+            type: String,
+            value: 'master'
+          },
+
           /**
            * The relative root for determining paths to demos and default source
            * detection.
@@ -532,7 +541,7 @@ <h2><span>[[active]]</span> <span class="version" hidden$="[[!version]]">[[versi
           // Rewrite gh-pages URLs to https://rawgit.com/
           var match = srcUrl.match(/([^\/\.]+)\.github\.io\/([^\/]+)\/?([^\/]*)$/);
           if (match) {
-            srcUrl = "https://cdn.rawgit.com/" + match[1] + "/" + match[2] + "/master/" + match[3];
+            srcUrl = "https://cdn.rawgit.com/" + match[1] + "/" + match[2] + "/" + this.branchName + "/" + match[3];
           }
 
           this._baseUrl = _baseUrl(srcUrl);