make it possible to register a mapper for dynamic arrays

a DynamicArrayFieldMapperBuilderFactory has to be implemented which returns a
Builder for a mapper that is able to handle dynamic arrays and creates FieldMappers and/or ObjectMappers for it
it's main usecase is mapping arrays explicitly as arrays. For this to work consistently even with dynamic fields
it is necessary to catch parsing dynamic array columns in the indexed sources.

If no DynamicArrayFieldMapperBuilderFactory is bound via guice, everythign is working as before.
allow for migration if metadata loaded from local gateway (filesystem)
Merge pull request #34 from crate/bm/dynamic_array_mapping

bm/dynamic_array_mapping