Add EventDispatcher to extend JobFlow easy

[tyx]

In order to extend Jobflow easily, we should add event system.

Events like :

• before execute method
• after execute method
• when publishing message
• when reading message
• ...

We need to keep jobflow core KISS but we need advanced stuff. So it will be the best way I guess

[tyx]

Add it in 4978de7efa444c42a1882dac48093124f9e803a0