Call `make init` on install
Use AIRSTACK_HOME env var during install
Change AIRSTACK_BUILD_* vars to _TEMPLATES

The “build” dir now contains the build artifacts
as developers would expect. Docker templates are 
now in .airstack/config by default. The cache 
dir is no longer needed.
Update Makefile template with /package
Add todo’s to Makefile
Set catchall target in Makefile template