Adding NEWS entry for plugins.

Also editing the plugin CHANGES somewhat.
Adding missing check that a plugin's API version matches what Bro defines.
Fix reference counting bug in Tag::operator=
Fix for redefining built-in constants.

Turns out redefs weren't visibile to core-land due to initialization
order.
Merge remote-tracking branch 'origin/master' into topic/robin/reader-writer-plugins
More polishing.