* Introduce an Attributes interface and have ImmutableAttributes and the AttributesMap implement it.
* make the attribute limiting a little clearer.
* tiny javadoc fix
* Rework to restore the Attributes class, and introduce a ReadableAttributes interface
* polish the javadoc a bit
* fix some broken javadoc
* revert example change; revert test name change
* javadoc de-escalation
* fix bad merge