Necessary extension to allow custom classloading to be plugged into myfaces in a server compatible way

Additions to the shared part and to the ServletContextListener are done to allow the hot plugging of custom classloading into myfaces.
This is done to allow the pluggability of scripting engines without having to add a custom classloader to our system. Classloaders are inherently problematic in server environments.

Theoretically anything can be plugged in, but for now we use it only to allow the pluggability of a groovy classloader.