Send the X-PJAX header with request (#80)

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

@@ -22,6 +22,7 @@ module.exports = function(location, options, callback) {
 
   request.open(requestMethod.toUpperCase(), location, true)
   request.setRequestHeader("X-Requested-With", "XMLHttpRequest")
+  request.setRequestHeader("X-PJAX", "true")
 
   // Add the request payload if available
   if (options.requestPayloadString != undefined && options.requestPayloadString != "") {