fix for repository paths that are not relative

- changes behaviour of get_file method for
  Alien::Base::ModuleBuild::Repository
- checks if the HTTP link passed to get_file in Repository::HTTP is an
  absolute URI

This changes the API for the Repository classes, but I've checked CPAN for possible subclasses and could not find any. There may be a better way to do this.

The specific package I made this change for is [libuninum](http://billposer.org/Software/libuninum.html).