DESTDIR support

This should resolve #39 

also noted a documentation error when I was reviewing the documentation in writing the test.

In addition to the unit test, I was able to build rpms in OBS for both Acme::Alien::DontPanic and Acme::Ford::Prefect

https://build.opensuse.org/project/show/home:platypus:Alien-Base
Actually... should that be a /e? It seems like perl-evaluating the env var isn't right?