Allow adding same classes multiple times

Fixes #2 

As discussed in #2 I still don't get why this PR is not valid. So let's talk code :p

My Drupal users may type
```
[classdiagram
\Drupal\Component\Plugin\ContextAwarePluginBase
\Drupal\Core\Plugin\ContextAwarePluginBase
\Drupal\Component\Plugin\PluginBase
]
```
and without this patch this results in
```
ID must be unique
```
which by no means is solvable for my Graph API module.

With this PR we get this beautiful UML
![no-id-must-unique-awesomeness-2](https://f.cloud.github.com/assets/371014/1965586/0341e188-82bc-11e3-8395-5b34e9c9da1c.png)
  
(This PR saves me a lot of hairs)