Callback symbols and transition callbacks by convention

[tlloydthwaites]

When building a workflow spec dynamically we can't specify procs, but we can specify symbols for methods we want to run.

This PR allows us to specify a symbol when calling on_entry and on_exit.

It also tries to call before_transition, on_transition, after_transition, and on_error as methods on the workflow object as well as using the standard procs.