add TextModel and extend models instead of having a flat list.

[jeroen]

• Extending the models isn't really needed yet, but easy enough: VideoModel > HTTPModel + RTMPModel. Let's do this when the first CDN-specific models are being written.
• A TextModel tends to add more issues than it solves: which text? description? So discarded.

Adding custom models is now very simple:

1. Write the model (implementing ModelInterface)
2. Add the model to the Model.as lists' loadModel.
3. Load a video with the 'file' and 'type=mymodel' flashvars.

If autodetection of a certain model is needed, the Controller.as' getModelType() function can be amended.