DOC: python-2.6 minimium now. Closes gh-526

Clean up ChangeLog and README.md to reflect these changes.
Remove credit from developers for individual changes to be consistent
with the 0.8.12 ChangeLog. Update summary and priority of items listed
in ChangeLog.
Merge pull request #538 from grooverdan/0.9-python-2.6-min

DOC: python-2.6 minimium now. Closes gh-526