Add dynamic overloads for:
- QueryFirstAsync
- QueryFirstOrDefaultAsync
- QuerySingleAsync
- QuerySingleOrDefaultAsync

This may change a lot in 2.x, but many people asking for them now so here they are. Unit tests also fixed for QuerySingleOrDefaultAsync.

Note: this builds on the netstandard2 branch because of the xUnit changes to go with VS 2017 15.3.