Add Table-Valued function support to EntityMapping.

[Freydoonk]

TableValuedFunction supporting was added. To did this the EntityMapping class was converted to an abstract class. TableValuedFunctionEntityMapping<T> and EntityMapping<T> which inherited the abstract EntityMapping were added.

SetDatabseName was addedd to EntityMapping configuration options. A new facility was added to specify a Database name to each entity while doing configuration.

[MoonStorm]

Thanks for sharing your work. I'm gonna try to find some time to review it. I have a couple of ideas myself to refactor the JOINs. We'll see what comes out.