Allow specifying the classloader to use for loading webapps.

If one uses a context.xml with a <loader> element to specify the class loader implementation to use with the tomcat7:run goal, this causes Tomcat to override the WebAppLoader carefully created by the plugin, without the
benefit of a properly configured class path.

The attached patch adds a <classLoaderClass> property to configure which class loader implementation to use. This is probably of limited utility as most classes will be loaded by the parent classloader anyway.