Add serializer parameters to docs, tests and examples

[woutdenolf]

Closes #339

There are 4 (de)serialization options and we were using only mentioning 2

Client
    |
    | task_serializer  (NEW)
    v
 Broker
    |
    | accept_content
    v
 Worker
    |
    | result_serializer
    v
 Result Backend
    |
    | result_accept_content  (NEW)
    v
 Client

The default is json which is why tuples got serialized as lists.