2.6-beta and 2.7 parallel development

Due to how our version tagging works and to prevent things destined
for 2.7 getting incorrectly labelled as part of 2.6-beta in CHANGES,
I'm going to start a dev/2.7 branch.

* Any new commits that should be included for final 2.6: merge to master
* Any new commits that are meant for 2.7: merge to dev/2.7

After a final 2.6 release is tagged, dev/2.7 can then be merged into master.

- Jon