More robust get_registry_record.

Previously if a record had a value of 'None', then get_registry_record would
fail with KeyError. This is now fixed.

I also added a more helpful error message that, in case you passed an
inexisting record name, prints a list of available records.

Lastly I threw some code around to make it prettier.
docs on how to get/set member properties
minor doc addition
Merge remote-tracking branch 'plone/master'