Open up support for arbitrary resource types

This is an updated copy of osgi/bindex#34 

In Apache ACE we need to be able to generate repository indexes that contain non-jar resources. At present repoindex does not support this because it assumes all resources are jars.

This patch introduces a new extension interface ResourceRecognizer that is repsonsible for delivering
the Resource implementation and/or sets the appropriate mime-type.

Based on that mechanism I have added default implementation for bundles and plain jars as well as json, xml and metatype files. In addition the default implementation provides identity and content for any type of file.