build: implement the setup.* files for non-merged devel spaces

Ok, this should address #6, I have generation of `setup.sh`, `setup.bash`, and `setup.zsh` into the root of the devel space, if the `--merged-devel` option is not used. In the case that the `--install` option is used, the generated `setup.*sh` files will just print out an error and not modify the environment, because each of the devel spaces were built on top of the previous packages' install spaces and that probably isn't what the user intended to do. Further checking and warnings can be done with #11.

@jbohren