Example implementation

Hey @eggplantbren 

Here's a little demo. Let me know what you think.

The file [demo.py](https://github.com/eggplantbren/DNestCompiler/blob/dfm/demo.py) builds a model something like:

```
theta ~ uniform(0, 1)
theta2 <- pow(theta, 2)
x ~ normal(theta2, 0.5)
```

Don't know if this is reasonable.

The two basic results are in the [demo_results](https://github.com/eggplantbren/DNestCompiler/tree/dfm/demo_results) directory.

Let me know what you think.