Save JSON configuration prevalue to database

As described in issue #34 (imulus#34) the Archetype configuration quickly exceeds the 2500 characters allowed by Umbraco prevalues.
This pull request implements a solution to the problem by moving the Archetype JSON configuration to a separate custom table within the Umbraco database, and solely using the Umbraco prevalue as an identifier to find the Archetype configuration in the new table.
The custom table gets created automatically when the package is installed, and dropped from the database when the package is uninstalled.