-generalised source retrieval code from builds.py, now resides in build_utils.py,
source_retrieval.py and new plugins. This functionality was tied into pkg
src downloading, but the new rez-install tool needs it in order to d/l and
update formulae repos.
-new entry point for downloading src is source_retrieval.get_source()
-NOTE existing builds.py is unchanged, but will need to be deprecated as there
is duplicate code, I have not had time to do this. pkg-specific code should
probably be put into a new 'build_package.py' file.
-added yapsy-based plugins for source retrieval - 'archive', 'git'. Hg not
done yet.
-added general plugin manager
-added zip support to archive downloader.
-added some py2.5 compatibility.