Add uprofiler plugin

Add plugin for the xhprof fork [uprofiler](https://github.com/FriendsOfPHP/uprofiler).
Can be enabled using `install_uprofiler_master` in your definition file.

This is a shameless copy of the [xhprof plugin](https://github.com/CHH/php-build/commit/d7617ff8464496b0e1993ac5969ba2e2e239e098) by @jtakakura, so all credit belongs to him.