Fix for GH dgrid issue #489.
Updated _StoreMixin to reset _columnsWithSet only when configStructure() is called,
rather than resetting it everytime _configColumns() is called because
_configColumns() is called multiple times for column configurations
containing subRows.
Add DOH tests for _StoreMixin to verify column.set() method now works with subRows.
Add _StoreMixin tests to test/doh/module
Fix silly mistake where require() was used instead of define()
Merge branch 'master' of git://github.com/brandonpayton/dgrid into columnswithset-reset
_StoreMixin: always initialize _columnsWithSet to reduce logic in save
_StoreMixin: tweaks to DOH test page
Update CHANGES.md to note fix to column.set for subrows/columnsets