Support for difference sets

I implement support for difference sets of database content.  This is
necessary if future versions want to provide base data such as sel_value
or seq_number entries.  Now you can add difference sets for a particular
database version to /WEB-INF/data/install/db-diff-set-#.sql where # is
the database version.  Such a difference set may contain any SQL
statement.  Depending on the stored database version in the config table
under the key "dbVersion" all difference sets up to the current version
are applied in grails-app/conf/BootStrap.groovy.