Preparing for producer overhaul

+ Maru::Client facilitates connection to a server, handling
authentication automatically.
- Old producers are gone.

In the future:

+ The subscriber will have a plugin API allowing it to be aware of
job content types.
+ One of the features included in the plugin API will be the ability
to implement producers.
+ Plugins will also be able to create hooks, allowing for the
possibility to create further jobs automatically based on the result of
previous jobs when they are received.
Migrate subscriber to proper configuration format