DATAREDIS-206 - Add support for TIME operation.

Enhanced RedisServerCommands to reflect time operation and added empty implementation to recent RedisConnection abstractions.
Added implementation of time() for Srp, Jedis.
Added UnsupportedOperation of time() for Lettuce and JRedis.
Added implementation for DefaultStringRedisConnection to delegate time() operation to native connection.
Added integration test to abstract base class. We need to add and ignore the test for those connections not supporting time command.

Original pull request: #34