Save JSON configuration prevalue as chunks

As described in issue #34 (imulus#34) the Archetype JSON configuration
quickly exceeds the 2500 characters allowed by Umbraco prevalues.
To circumvent this problem we'll chop the Archetype JSON configuration
into several prevalues of max 2500 characters before saving the datatype
to the database
To anyone testing this... the max chunk size is defined as the constant chunkSize within the Chunkify method of the ArchetypePreValueEditor class. The easiest way to test this might be to lower the max chunk size in a custom build, to have the JSON configuration chopped into several smaller chunks without having to create gigantic Archetypes :)