Add the ability to host external models in the demo

[schmmd]

Presently many users build models in their own source repositories. If we wanted to host such a model in the AllenNLP demo currently we would need to port it into http://github.com/allenai/allennlp itself. However this is often impractical or even unwanted.

One potential strategy for serving external models is to containerize each model behind a rest interface. This would also allow the demo to scale easier and it would allow us to isolate bad models (such as the model that's leaking memory in #37).