fixes boot-clj/boot#22 -- add on-env! watcher to :tgt-path

Hey guys, here's my attempt at fixing boot-clj/boot#22. It changes the registry api a bit so I'm not sure if that's desired, but hopefully this helps in the right direction. As for testing, I've just done top-down tests with one of my repos. Pls let me know if ya'll have more detailed testing procedures to follow. Cheers :)