Add generic Linux support

[matyasselmeci]

Since Pelican is a statically linked binary, it would be useful to download the tarball and extract the program into /usr/local/bin if we're not on a platform that has one of our supported package managers.

It might be useful to add a download() function that automatically used curl -LO or wget or busybox wget depending on which one was available. The busybox:latest image would be a good one to test with.