Allow Zepto to work with jQuery in noConflict mode

* Check that window.$ is undefined rather than whether $ is a property
 of window.
* jQuery in noConflict mode sets window.$ to undefined which was causing
  Zepto to not be assigned
Make syntax more consistent
Merge pull request #796 from sharethrough/master

Allow Zepto to work with jQuery in noConflict mode
Add Android 4.3/Nexus 4 detection test
Add detection for Safari
Remove spurious console.log, fixes #803