Provide helper to delete primary key without knowing its name

[ashmind]

When improving the database that uses autogenerated primary key names, working with them currently requires custom SQL.

As there can be only one primary key on a table, Delete.PrimaryKey()... should work without providing the name.

[tommarien]

@ashmind Do i smell a pull request coming ;)

[ashmind]

@tommarien Well, I can do one for SQL Server, but I do not have Oracle etc to test other generators.

[fubar-coder]

Seems like a candidate for a new expression convention.