Add optional label to individual doodles

[CedricReichenbach]

This label should be displayed as meta information, similar to the class name.

API details are yet to be defined, some ideas:

• The Doodleable interface has another method, like getDoodleLabel
• Introduce another interface, like LabelledDoodle with one method returning a string
• Doo.dle("John Doe", contact);