Serialize indexed annotations as JSON

To support Java classpath libraries whose ```TreeMap``` has a serialization that is incompatible with Oracle Java's (Android, I am looking at you), we have to decouple SezPoz from the vagaries of ```TreeMap```'s serialization.

To maintain backwards-compatibility, we still accept ```.jar``` files whose annotations were serialized by a previous SezPoz version.
 
This is intended to fix issue #6.