[WIP] Improving initialization

[tshort]

Trying to make successive runs consistent. Added tests to capture this.

Added an experimental initialize! method that uses JuMP to initialize the SimState (issue #28). Things are still broken. It's a work in progress.

[tshort]

I've got a working initialize!. The issues are:

• It only works for scalar Unknowns, not vectors.
• It uses JuMP, and this increases load times. I tried using @require to make it optional, but I couldn't get it to work. Overall, I think this is okay. We'll have package caching sooner or later. JuMP is an uber-cool package.
• It doesn't work after events, so if you have events in a model, you don't want to use sim(m, init = :none).

This pull request also adds some general tests for model simulation and compares DASSL and Sundials results. We should add more of that sort of thing.

Unless I hear complaints, I'll probably merge this as is. initialize! isn't perfect, but it might help for models where the built-in initialization doesn't work, and it might be a starting point for more work here.