Add basic compilation and model opening support to Tortoise

This PR gets rid of the hardwired Tortoise model and allows general compilation. You can now type stuff into the text editor and it will be compiled by Tortoise and be runnable by the client.

Limitations:

- No error reporting (this was already a problem with Tortoise on commands)
- Opening a model from the drop down should theoretically work, but only if the model does not declare any globals via widgets... or have special dependencies on the dimensions... or use any netlogo features not yet supported by Tortoise (obviously)...
- World dimensions are aren't read from the nlogo file contents

This code just uses NetLogo's nlogo parsing code when opening a model, so getting things like widget-globals and such shouldn't be hard.

I'm partially just opening this pull request so some other eyes see this design before I get too far with it. I tried to stick with the architecture put in place by @TheBizzle as much as possible.

@frankduncan, this addresses part of what you mentioned on the mailing list.