[WIP] Merging pydy-code-gen and pydy-viz

This uses the method described here:

http://nuclearsquid.com/writings/subtree-merging-and-you/

- [x] Merge pydy-code-gen into a subdirectory
- [x] Merge pydy-viz into a subdirectory
- [ ] Merge pydy_examples into a subdirectory
- [ ] Make changes in the merge-codegen branch to get tests passing and docs correct