Added checkstyle to pom.xml
Lots of minor things have been changed all over the plugin in order to
fix violations of the defined checkstyle rules. The UUIDFetcher is
currently not covered since it will be removed soon due to licensing
problems.
Additionally, JOOQ is now shaded into an internal package and the pom
has been reformatted.
Removed PaginatedResult in favor of a new, modular StringPaginator
Integrate flyway for database migrations
At the moment the integration covers future migrations only, as
migration of legacy layouts is still handled by the custom migrators.
Using the automatic flyway migration could enforce uneeded dependencies
on old propperties (e.g. table-name) that are needed for the conversion,
but not for any new behaviour.
Additionally handling the database could be problematic as the new
layout should have its own database in most configurations - the old,
single table layout however could have easily been placed in a database
common for minecraft plugins (as was indicated in the default
configuration).

Fixes #16.