[SPARK-1377] Upgrade Jetty to 8.1.14v20131031

Previous version was 7.6.8v20121106. The only difference between Jetty 7 and Jetty 8 is that the former uses Servlet API 2.5, while the latter uses Servlet API 3.0.

Author: Andrew Or <andrewor14@gmail.com>

Closes #280 from andrewor14/jetty-upgrade and squashes the following commits:

dd57104 [Andrew Or] Merge github.com:apache/spark into jetty-upgrade
e75fa85 [Andrew Or] Upgrade Jetty to 8.1.14v20131031