Dynamic format for repository reference numbers.

### Requirement
The format how the reference number is composed and displayed, differs from customer to customer.

Currently there are two popular formats:
* **dotted**: After every level the particular prefixnumber is seperated by a point
![bildschirmfoto 2013-10-03 um 09 20 25](https://f.cloud.github.com/assets/485755/1259941/6adc1b72-2bfc-11e3-896c-7c55ec48cb7f.png)

* **grouped by three**: Every level is represented by a single character. The prefixes are grouped by threes and separated by a dot. In this format only prefixes from 0-9 per level are possible.
![bildschirmfoto 2013-10-03 um 09 31 21](https://f.cloud.github.com/assets/485755/1259980/d716f7ca-2bfd-11e3-97a1-b0f4feae393e.png)


### Technical implementation, Todos
- [ ] All formats could be registered as named utilities.
- [ ] The currently active format should be configured by a `plone.app.registry` entry, which allows to select all registered formats.
- [ ] The starting point for reference prefixes should be configurable in the registry as well.
- [ ] The reference adapter (https://github.com/4teamwork/opengever.core/blob/master/opengever/repository/reference.py#L26) should use the utility which is selected in the registry.
- [ ] The Title-accessor for repositoryfolders(https://github.com/4teamwork/opengever.core/blob/master/opengever/repository/repositoryfolder.py#L103) should use the utility as well.