Send the PJAX header with request

Make it more compatible with the jQuery PJAX and lets the server choose to optimize the response. Sending the list of selectors would be nice, too.

lib/request.js

@@ -19,6 +19,7 @@ module.exports = function(location, callback) {
 
   request.open("GET", location, true)
   request.setRequestHeader("X-Requested-With", "XMLHttpRequest")
+  request.setRequestHeader("X-PJAX", "true")
   request.send(null)
   return request
 }