Switch Redis library to to StackExchange.Redis

- [x] Switch to [StackExchange.Redis](https://github.com/StackExchange/StackExchange.Redis) :clap:.
- [x] Use `NetDataContractSerializer` as default serialzier, but allow it to be customizable.
- [x] Support .NET 4.0 and 4.5 releases.
- [x] Bring over parts of ServiceStack.Redis to support distributed locking?
- [x] Update README with changes & contributors.