Session manager factory support

This PR adds the ability to specify a session manager factory on the command line within the standalone war. Using this patch, in conjunction with a matching memcached-session-manager patch, you can use the memcached-session-manager with start your standalone project like this:

````
java -Dmsm.memcachedNodes="n1:localhost:21211" -jar standalone.jar \
   -sessionManagerFactory de.javakaffee.web.msm.MemcachedBackupSessionManagerFactory
````

The memcached-session-manager patch can be found here:

https://github.com/magro/memcached-session-manager/pull/33