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 "plugin"
- jar files embedded in plugins (in a /lib folder) are automatically unzipped to a folder named <plugin>-libs and added to the extension classloader (required write permissions to the plugins folder)