Add an option to set a timeout for XHR requests (#95)

Closes #24.

lib/request.js

@@ -15,12 +15,17 @@ module.exports = function(location, options, callback) {
     }
   }
 
+  request.ontimeout = function() {
+    callback(null, request)
+  }
+
   // Add a timestamp as part of the query string if cache busting is enabled
   if (this.options.cacheBust) {
     location += (!/[?&]/.test(location) ? "?" : "&") + new Date().getTime()
   }
 
   request.open(requestMethod.toUpperCase(), location, true)
+  request.timeout = options.timeout
   request.setRequestHeader("X-Requested-With", "XMLHttpRequest")
   request.setRequestHeader("X-PJAX", "true")