Add support for getting results as DataSets and DataTables

I realize that adding support for DataSets and DataTables is almost like taking a step backwards in a way.

However, there are many of us who unfortunately still need to use DataSets/DataTables to work with various third-party libraries. With Dapper's excellent parameter handling and support for lists, there is a huge advantage to being able to use Dapper like this.

I added `QueryDataSet()` and `QueryDataTable()` extension methods along with Async versions and some tests. A reference to System.XML was required, but I don't see why that would cause any problems.