Add support for automatic persistence

[joaomdmoura]

In order to help ppl easily use Machinery with more complex applications, like Phoenix, Machinery needs to provide support for automatic persistence. Right now people need to use after_transition/2 to store their structs.

Would be nice to have some sort of config that ppl can use to set specific storages, so a user might want to use ETS but others might want to use Ecto + Database (for Phoenix apps).