Don't hardcode the alias in the filters.

This way you can add more complex filters like '(someField IS NULL)' for a BooleanFilterType and it'll check if the result of this expression is true or not.
Handy to add a filter on the existance of a certain variable in the database.
Merge pull request #78 from Kunstmaan/feature_less_restrictive_filter_23

Don't hardcode the alias in the filters.