Consider adding a MessagePack option (instead of JSON) for storing complex data

[bkirz]

Qless currently uses JSON to store job history, data, tag lists, and failure information. Since Redis exposes a messagepack library to EVALed LUA scripts, it would be nice to have an option to use it for space saving reasons. Adding it as an option lets users decide if they want to save space with msgpack or be able to browse their redis keys easily without using the qless-core API.