add check for navigator (insin#83)

* add check for navigator

This enables server side rendering

* fix eslint errors

* changes per feedback

`typeof navigator !== 'undefined'` otherwise strict mode will still throw
`return 'onKeyPress'` as a fallback in case someone is removing navigator from the global scope

Author: aesopwolf

src/index.js

@@ -236,9 +236,12 @@ var MaskedInput = React.createClass({
   },
 
   _keyPressPropName() {
-    return navigator.userAgent.match(/Android/i)
+    if (typeof navigator !== 'undefined') {
+      return navigator.userAgent.match(/Android/i)
       ? 'onBeforeInput'
       : 'onKeyPress'
+    }
+    return 'onKeyPress'
   },
 
   _getEventHandlers() {