Refactor for a clearer directory structure

[vincekurtz]

This follows @amcastro-tri's suggested layout for the most part, with some naming changes make more sense to me.

Now the structure is

• examples
  • acrobot
  • airhockey
  • ...
• optimizer
  • test
• scripts
• tools
• utils

For example, the main solver is in optimizer instead of traj_opt since that's a bit more specific (this whole repo is about traj_opt). I also got rid of some of the tiny old examples that we just used for development/debugging.

---

This change is