Add an ObjectMapper shorthand

[cyberdelia]

Add an ObjectMapper shorthand, allowing a less verbose initialization:

ObjectMapper objectMapper = new MessagePackMapper();

Rather than:

ObjectMapper objectMapper = new ObjectMapper(new MessagePackFactory());

It's similar to other dataformat implementation as well, see: https://github.com/FasterXML/jackson-dataformats-binary/tree/master/protobuf#creating-objectmapper

[komamitsu]

Looks good. Thanks! 🍷