create generic datatype support

[hohwille]

As discussed in issue m-m-m/util#123 we want to create generic support for custom datatypes. Therefore we have to hook into hibernate (HibernatePersistenceProvider) or we would need a list of all datatypes and register UserType or CompositeUserType instances. As latter case has the danger that we could replace or collide with hand written UserTypes or AttributeConverters I do not see this as a good option.