[ZDWB-67] Created the plain DatabaseExistsAssertion which can now replace the original MySql and SqlServer versions since the responsibility for determining the existence of a database is now defined by DatabaseInstance and is implemented for MySql, SqlServer and PostreSql.  Also wrote a new "FixtureCreator" with a fluent API for making construction of XML fixtures much more verbose.  Replaced all uses of XmlBuilder for resource fixtures with FixtureCreator.  All unit tests pass.