Adding the Rössler attractor as a Chebgui demo

The Rössler attractor is an example of chaos even more simple than the Lorenz attractor. (Lorenz has two nonlinear terms; Rössler only one.) The IVP solver allows it to be solved on a large domain. Here is a 3D plot of the result on [0, 100]:

![rossler](https://cloud.githubusercontent.com/assets/913747/4687373/a77b1e48-5653-11e4-8eb2-ab2b55b527c3.png)