GPCACHEREDIS-3: Add TTL support to grails-redis-cache plugin.

An initial attempt at implementing a TTL configuration for the Grails Cache Redis Plugin.

The following would provide the ability to globally configure a TTL for all Redis caches.  I would much prefer the ability to set this at a per cache level, but had some trouble making it work with current cache config parsing.  Would be happy to add that ability after a discussion of how best to accomplish.

Thoughts?  Concerns?