Make app registry location configurable

Using environment variables:
- HUE_APP_REG_DIR
- HUE_PTH_DIR

A link from build/env/lib/python*/site-packages/hue.link.pth will
point to <HUE_PTH_DIR>/hue.pth.

If the app registry is in a non-default location, all Hue
commands will need to be executed with HUE_APP_REG_DIR set.

relinking will be a manual process for now.