Add additional topics to publisher for monitoring AttentionAllocation

- Add new subscription topics useful for monitoring the AttentionAllocation system:
    - **addAF**: Triggered whenever the AttentionValue of an atom changes, if the new STI value
is within the AttentionalFocus boundary and the old STI value is not.
    - **removeAF**: Triggered whenever the AttentionValue of an atom changes, if the old STI value
is within the AttentionalFocus boundary and the new STI value is not.
- Add fields to AtomSpace publisher JSON message types: 
    - avOld
    - avNew
    - tvOld
    - tvNew
    - handle
    - atom
- Updated unit tests & documentation:
https://github.com/opencog/opencog/blob/master/opencog/persist/zmq/events/README.md