Avoid exception when unable to match UA

Make detect() return an empty object if it hasn't matched any known browser, so that graded browser support (and any external code using this library) doesn't raise a "Cannot read property ... of undefined" exception

src/bowser.js

@@ -90,6 +90,7 @@
         seamonkey: t
       , version: ua.match(/seamonkey\/(\d+(\.\d+)?)/i)[1]
     }
+    return {}
   }
 
   var bowser = detect()