Merge branch 'release/2.0.4' into develop
Conform with formencode - http://bit.ly/KDpyra.
Removed unreferenced variable.
PEP8.
Updated tests to work against tw2.core-2.1.0
PEP8 cosmetic.
Test for setting property to 0.
Allow integer assignment to 0.

  - Reported and patched by Juraj Variny.  Thanks!
  - Fixes http://bit.ly/M43f1D
Use the DbCheckBoxList widget for the one to many relations
Add support for the many to many relations
Add 'required' logic for the relation properties
Add support for the one to one relations
Modify DbSelection widgets so the primary key can have any name
First cut of DbLinkField
DbFormPage - only filter by primary key columns
Tidy up the docs a little
Fix to the README.
s/entry_point/entry_points/... Thanks @janis_!
Fix failing tests related to selection field changes
Tweak tests so that DbFormPage only loads using primary key