Added transaction handling and corrected DateTime quoting format string for Jet

Added transaction handling to Jet (the mdb file was locked after running the migrations the way it was, because the connection was never closed). Also added a finally { ... } clause to MigrationRunner to ensure closing of the connection, because VersionLoader.LoadVersionInfo() reopens it after the transaction has been committed.

JetQuoter.cs contained a wrong format string for DateTime, resulting in the insert into VersionInfo failing. Has this ever worked? Probably noone uses Access. Good for them! :-)