Add checks for XHR redirects

Fixes #7

This checks for redirects by looking for the following, in sequence:
- XMLHttpRequest.responseURL
- the X-PJAX-URL header (like jquery-pjax)
- the X-XHR-Redirected-To header (like Turbolinks)

lib/request.js

@@ -15,6 +15,11 @@ module.exports = function(location, options, callback) {
     }
   }
 
+  request.onerror = function (e) {
+    console.log(e)
+    callback(null, request)
+  }
+
   request.ontimeout = function() {
     callback(null, request)
   }