DATAREDIS-73 - Add support for spring managed transactions.

RedisTemplate now supports enabling of transaction support, which is disabled by default.

In case of enabled transaction support the RedisConnections will be bound to the current thread during ongoing outer transactions. We call MULTI at the beginning and depending on transaction state EXEC or DISCARD at its end.

To support read operations during the transaction we wrap a proxy around the bound connection piping read operations to a new (non thread bound) connection obtained by the underlying RedisConnectionFactory.

Transaction support is available for jedis, lettuce and srp, while had to be skipped for jredis due to the lack of support for MULTI.

Original pull request: #64.