Added new across-the-board DataStore col param 'is_nullable' - fixes #33

Since nullability is something that is understood by RapidApp in
general, decided to add a full meta/column param (i.e. available within
javascript DataStore, etc) 'is_nullable'. This setting now overrides and
sets 'allowBlank' within the add/edit forms, but is not yet used anyplace
else (although it is now available in the Public API). Moving forward,
we will start using this other places where we need to take programatic
action on column nullability which will be more consistent.