* Vendor ordereddict for python2.6 only
This commit substitutes the custom module 'ordereddict_backport' with
the more known 'ordereddict' and vendors it only for python 2.6. Other
supported versions of python will use 'collections.OrderedDict'.
* Use absolute imports also for python 2.6
See PEP-328 for more information on the subject
* Added provenance of vendored ordereddict