Allow use of buffers instead of strings with Redis

[caitisgreat]

redis & redis-parser defines the use of Buffer in Redis, but disqueue-node isn't configured correctly to use buffers and always assumes we don't want to use them. This pull request allows a user of this library to use buffers as a return type.

From redis-parser docs https://github.com/NodeRedis/node-redis-parser#options

returnBuffers: boolean; optional, defaults to false

but then redis uses this

From redis docs https://github.com/NodeRedis/node-redis#options-object-properties

return_buffers	false	If set to true, then all replies will be sent to callbacks as Buffers instead of Strings.