First stab at updating the CHANGELOG for 2.0

Probably more to do here..
[#523] Initial clean of extensions.rst
[#523] Edit writing-extensions.rst part I
CHANGELOG tweaks suggested in pr
Add some more stuff to the changelog
[#534] Update configuration flags' documentation
[#534] Reorganize configuration flags' order in the docs
[#523] More writing extensions updates
[#523] Toolkit update and other fixes
[#523] Merge extensions and writing extensions
[#523]  Remove erroneous section on plugin ordering
[#531] Update data viewer docs for CKAN 2.0
Merge remote-tracking branch 'origin/master' into 531-data-viewer-docs
[#535] Move error messages related to test to a separate page.

The idea is to write the common errors that might happen closer to where they're
more common, so you have the information nearby when you need it.

This works well for test errors. For install/upgrade, as we have two methods,
we can't simply add to the same page. I've changed the index so the common
errors page is under the Installation section. Only installation/upgrade
issues should be added there.
[#534] Add documentation for some configuration flags
[#534] Remove useless indexes in configuration page
[#534] Minor styling adjustements
[#534] Remove unused configuration flags
[#523] Improve docs based on Sean's feedback
[#523] More Extension docs reworking