Drop third-party payload builders

[jchambers]

The Jackson and GSON payload builders really don't produce very much value, and certainly not much relative to their maintenance burden. This change discards third-party payload builders as Pushy modules, but adds payload builder methods so callers can serialize payloads using the tool(s) of their choice.

For example, what was previously done with a dedicated payload builder subclass can now be done with something like:

final ObjectMapper objectMapper = new ObjectMapper();

final String payload = new ApnsPayloadBuilder()
    .setAlertBody("Example!")
    .build(objetMapper::writeValueAsString);

Ultimately, this should give end users more flexibility and also make Pushy easier to maintain.