render_model with/only - FINAL REVIEW

Resolves armstrong/armstrong#52 so that `render_model` can use optional `with` and `with only` parameters. Also:
- adds new tests 
- updates the Readme
- fix: fails silently on missing templates except in TEMPLATE_DEBUG (the Django way)

We've been using this at TBC/CIR for the last year. This is finally a proper official version. Now the template tag mirrors `include` and can do things like:
```
{% render_model model_obj "template_name" with additional="data" %}
{% render_model model_obj "template_name" with isolated="context" only %} 
```