Adds support for Count operator translation to relational.

This could be flattened to
 
``` SQL
SELECT COUNT(DISTINCT  c.[Address], c.[City], c.[CompanyName], c.[ContactName], c.[ContactTitle], c.[Country], c.[CustomerID], c.[Fax], c.[Phone], c.[PostalCode], c.[Region])
FROM [Customers] AS c
```
Not necessarily a ton better but in my experience additional nesting makes things harder for both humans trying to read SQL and certain query optimizers trying to produce fast query plans :smile:

Perhaps worth having a bug tracking un-nesting for this case?