Clean and organize "using" statements and unnecessary dll references

There are a lot of unused using statements across the Apache.NMS.ActiveMQ solution. As they just add some unnecessary noise to the codebase, we should remove them. We can also take the opportunity of such effort and organize them.
Also, I found some unnecessary referenced dlls (System.Xml in the vs2008-activemq project, System.Data, System.Xml in the vs2008-activemq-test project)