Port pelican to python 3.

Stays compatible with 2.x series, thanks to an unified codebase.
Update the "how to contribute" docs with py3k info.
Revert previously erased changes
remove py2.6 support
Annotate py3k code when needed.
Use the en-us locale for functional tests
Merge branch 'master' into py3k
Merge branch 'master' of github.com:getpelican/pelican