Moved external serializers to their own libraries

Hi I figured it would be best to keep the base package as light of dependencies as possible.
To do so I've moved all serializes with external dependencies to their own libraries.
This way the base package will only have StackExchange.Redis.strongname as dependency.

People who want Protobuf, JSON.NET, FastJSON and whatever other serializer will download those separate packages and dependencies as needed.

I've also cleaned the connection manager to better fit StackExchange.Redis. Connection strings can have multiple servers so I've removed the Host and Port stuff.

Cheers,
Fabian