added Kinetic level pixelRatio property that can be used to override the detected pixel ratio. Also fixed up a pixel ratio related test for FF. added Kinetic.UA for user agent detection

test/runner.js

@@ -3,6 +3,8 @@ var assert = chai.assert,
     kineticContainer = document.getElementById('kinetic-container');
 
 Kinetic.enableTrace = true;
+// make sure pixel ratio is 1 or else the tests will fail on devices with retina display
+Kinetic.pixelRatio = 1;
 
 function addStage() {
   var container = document.createElement('div'),