Adding Support for Couchbase as Caching Infrastructure.

This changeset brings in support for Couchbase Server 2.0 as
the caching layer. The interface is identical to the memcached
one, but using ext/couchbase allows to take advantage of the
automatic cluster discovery and failover functionality provided.
Merge pull request #236 from daschl/couchbase-cache

Adding Support for Couchbase as Caching Infrastructure.