Feature: callbacks on states and transitions

[OskHa]

The purpose of this pull request is to make it painless to display callback informations for states and transitions. Personally i use this to extend the information on my graphviz graphs.

• extended the interfaces
• added method on state machine to list all callbacks
• added method on states and transitions to list their callbacks
• global callbacks are ignored

[RonRademaker]

This PR breaks a lot of backwards compatibility by adding methods to an interface and dropping support for older PHP versions. Is there no way to accomplish these things without?

[OskHa]

i will have a look on that over the weekend.