DATAMONGO-881 - Allow custom conversions to override default conversions.

User provided converters are now registered *after* the default converters to make sure they enjoy precedence over the default ones. 

This is achieved by inverting the order of converters after the conversions have been registered. This is necessary as the registration order for convertible pairs is different from the one of the converters. For the pairs, earlier registered instances take precedence, while for the actual converter instances, instances registered later trump ones registered before.