Improved plugin-loading

improved plugin-loading: 
- when loading plugins, look for soapui.ext.plugins system property, then look for root, default to just "plugins" 
- plugin filename no longer has to end with "plugin.jar" - it just has to end with ".jar" and contain the word "plugin" 
- jar files embedded in a /lib folder in a plugin are automatically unzipped to a folder named <plugin>-libs and added to the extension classloader (requires write permissions to the plugins folder)