Drop Schema instead of migrating.

DBFlow Version: 4+
Issue Kind: Question

Description:
Sometimes it is not necessary to migrate data from version to version. For example models have significantly changed and we don't need locally stored data on upgrade as we will refresh it from the backend anyway. It would save us a lot of time if we could just drop my schema and let dbFlow generate a new DB when the versions change....instead of spending a lot of time writing migration scripts.