Add support for PubSub

Motivation:

One of the great features of Redis is being able to subscribe and receive messages published to specific channels as a way of acting as a message queue for processing jobs.

PubSub requires a specific understanding of the connection model that can only be implemented directly in this library.

Modifications:

• Add: RedisPubSubHandler to sit in front of RedisCommandHandler to manage subscription callbacks and Redis registration
• Add: publish and the pubsub commands
• Add: addPubSubHandler extension to NIO.Channel
• Add: Type-safe String wrapper of RedisChannelName for PubSub methods
• Add: pubsubSubscriptionNotFound error case
• Add: isSubscribed property to RedisConnection
• Add: availableConnectionCount and leasedConnectionCount properties to RedisConnectionPool
• Add: Metrics for PubSub
• Add: makeNewPool factory method to RedisConnectionPoolIntegrationTestCase
• Change: RedisClient to require methods for PubSub management, as they are intrinsicly tied to the client's connection model
• Change: Parsing of PING response for handling special case in PubSub mode
• Rename: ActiveConnectionGauge to RedisMetrics.IncrementalGauge

Result:

Developers will now be able to use Redis in PubSub mode with both connections and pools.

This resolves #6 (closed)