Use MultiFieldMapper as base for the core-mappers

Using MultiFieldMapper as a base class for the other core-mappers will make everything to work and we got the best of two solutions (adding all properties for mapping into MultiFieldMapper (#883) and depricate the MultiFieldMapper) see #873.

This change should not be any breaking change and we are still supporting version of ES (if it exists) that still sending out the "multi_field" as the mapping type.