Moved DatabaseQueryTransformRule to separate file

Hi Colby,

again, to facilitate implementation of a *fo-dicom* "portable class library", I have a new pull request.

This time I have moved one of the `IDicomTransformRule` implementing classes from the file *DicomTransformRules.cs* file to a file of its own. The class in question, `DatabaseQueryTransformRule` is the only class in this file that uses the `System.Data` namespace (thereby violating portable limitations). By moving this class to a separate file, I am able to exclude this class specifically in the portable library while maintaining all other `IDicomTransformRule` inheriting classes.

I hope you are OK with this modification. If you consider the new file to break any naming rules, please feel free to change the file name as you wish :-)

Best regards,
Anders