Use & instead of | (#4)

The intention of this snippet is to return true when the primary mouse
button is released. For that, a bitmask should be applied to only check
the primary button. I mistakenly used bitwise OR instead or AND, and as
a result the event.buttons check always returned false (`!1 == false`).

grab-to-pan.js

@@ -220,7 +220,7 @@ var GrabToPan = (function GrabToPanClosure() {
       // http://www.w3.org/TR/DOM-Level-3-Events/#events-MouseEvent-buttons
       // Firefox 15+
       // Internet Explorer 10+
-      return !(event.buttons | 1);
+      return !(event.buttons & 1);
     }
     if (isChrome15OrOpera15plus || isSafari6plus) {
       // Chrome 14+