added tests for CustomDTypeModel and Delegate.
Splitted UserRole into more constants used by the models (DTYPE_ROLE, DTYPE_CHANGE_ROLE, ...).
Refactored code (especially for ColumnDtypeModel.setData)
fixed merge conflicts
refactored code, added more tests, extended documentation
return None instead of raising an error
Added missing tests for TextDelegate
removed assert checks in production code
Merge pull request #3 from quiqua/tests_refactoring

Refactored code and added more tests