Added ability to fake packages on import.

- This is used in creating the docs, since Sphinx is in cPython
  and can't do the Java imports.
- Hopefully it will stop the docs from breaking so much
- I'm still using the dummy folder for some of the non-Nengo java imports,
  since I haven't figured out a good way to fake `from pkg import *`. This probably
  isn't possible, so really we should remove all imports of this type.