WL: standardize on python2.6

python 2.6 is ubiquitous now on all platforms except for RHEL5. Even RHEL5 we can depend on python26 fairly reliably via EPEL for all plugins except postgres - where an appropriate postgres connector is not available.

python 2.6 has a lot of advantages, namely contextmanager and functools that would make a lot of weird error handling in holland much cleaner. Also backports of useful packages like concurrent.futures is pretty clean in python 2.6, so we can make some of the parallel processes in holland 1.1+ much cleaner as well.

python 2.6 also makes integration with sqlalchemy and a proper backup catalog database backend much easier. This allows us to track metadata more cleanly than we are able to do today.

This should only be targetted at 1.1+ and will involve some significant changes to the code base to take advantage of python 2.6.