add a helpful response to SearchIndexError exceptions raised within the action api
[#835] return str(exception) on SearchIndexError in acton API
[893] Allow passing the engine to engine_is_pg and engine_is_sqlite so that can do the checks even if ckan uses a db which is different from the one the datastore uses
[#945] Upgrading docs reorganization.

Reorganise the upgrading and releases docs. We now have a single
Upgrading page that includes info about the different types of release
and the release process, and links to the various instructions for
upgrading CKAN.

Fixes #945.
Add 2.0.1 release date to CHANGELOG
Merge branch '945-merge-upgrading-docs'
Merge branch '835-helpful-action-api-search-index-error' of git://github.com/open-data/ckan into open-data-835-helpful-action-api-search-index-error
Merge branch 'open-data-835-helpful-action-api-search-index-error'
Merge branch '893-datastore-postgres-checks-fail-if-different-dbs'
Merge branch 'master' into 547-plugins-love

Conflicts:
	ckanext/datastore/plugin.py

    minor change to new_resource_show