Draft: Allow more customization and add sample document

1. Numbers are now hidden if it's exactly one Supervisor.
   `\ThesisSetSupervisors{Just One}` leads to:
   "Referee: Just One"
   `\ThesisSetSupervisors{{First One},{Then Another}}` leads to:
   "1. Referee: First One\\2. Referee: Then Another"
2. It's now possible to override the label if your thesis type
   needs a different one.
   `\ThesisOverrideSupervisorLabel{Betreuer}` changes the label:
   "1. Betreuer: First One\\2. Betreuer: Then Another"

Use `\ThesisSetType{Bachelor}` or similar to set the exact kind of thesis
you're writing. The class options `bachelor` and `master` have been
removed.