Migrated to StackExchange.Redis

Migrated to StackExchange.Redis.

Refactored how the connection is managed as StackExchange.Redis recommends to reuse the connection over multiple operations and connection management is mostly transparent.

Unit test "ItemExpires_Tag_RemovedFromCache" fails but it also fails on the Booksleeve version ("*:expired" subscription is never triggered in Redis 2.8.17).

Cheers!