Constructed a factory for use with the tomcat-maven-plugin standalone war functionality

This factory can be called from the tomcat standalone bootstrap code to use memcached-session-manager within your standalone project. I only implemented it for TC7 because the standalone war support is only for that version of tomcat. When using this factory, you configure msm with system properties. I could imagine this being improved with models.

Using this patch, in conjunction with a matching tomcat-maven-plugin patch, you can start your standalone project like this:

````
java -Dmsm.memcachedNodes="n1:localhost:21211" -jar standalone.jar \
   -sessionManagerFactory de.javakaffee.web.msm.MemcachedBackupSessionManagerFactory
````

I will cross reference to the tomcat-maven-plugin PR as soon as I create it.