Duplicate key detection does not work for (simple) Trees

[cowtowncoder]

(note: continuation from #50)

So: enabling JSON.Feature.FAIL_ON_DUPLICATE_MAP_KEYS does not make tree builder enable duplicate checking.

[cowtowncoder]

Alas, this can not be currently implemented because JacksonJrsTreeCodec does not have access to configuration information -- its readTree() method is only passed JsonParser but nothing else.

Not sure what would be the best way to tackle this: one option would be to require codec to be configured separately, but that does not seem optimal.