AS7 security 0.5 backport

Pretty simple cherry-pick of commits from master.

Conflicts/Differences:
* Implementation on master uses Guava 14's `BaseEncoding` class to turn a byte[] into a hex string. Brooklyn 0.5 is on Guava 13, so I've used Commons' `Hex` class instead.
* Needed to use `BasicAttributeKey` rather than `ConfigKeys.new...`