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
- [x] Merge pydy_examples into a subdirectory
- [x] Make changes in the merge-codegen branch to get tests passing and docs correct
- [x] Add notes to the READMEs in pydy-code-gen, pydy-viz, and pydy_examples to let people know the development has moved.