Jobdata transformers (WIP)

There is this emerging pattern that shows up every time I use Pheanstalk, where I send over composite data and have to serialize it in PHP or JSON, but manually doing it all the time.

Till now I had my custom extension to the original Pheanstalk class to achieve this, but I'd consider it may be a fine addition to the core.

Attached are a couple of lines changed/added to illustrate the point, and if it's a desired feature for you as well, I'll continue to add tests and what else may be required to have a good PR :)