SpringMiltonFilter now supports Spring @Configuration annoted classes

Just to let projects without any XML configuration use SpringMiltonFilter the same way.
It is now possible to use SpringMiltonFilter with a full Java Spring configuration and specify the class that defines Milton related beans through an init parameter called 'contextConfigClass'.

The old behavior remains the same: if contextConfigLocation is provided, try to load its content, otherwise, try to find a file named 'applicationContext.xml' in the classpath.

If nothing worked, it now just uses the parent context, which may be enough if all the beans are defined herein.