EF support improvement

SqlFunctionGenerator appended:
- PostgreSQL's Functions/StoredProcedures can be called through EntityFramework's ObjectContext.ExecuteFunction by this change.
- Currently we get ArgumentException because NpgsqlServices.TranslateCommandTree doesn't support DbFunctionCommandTree as an input.

Changes for NpgsqlSchema.ssdl
- I appended missing Association/AssociationSet elements in ssdl. They tell table's relationship defined in PostgreSQL, to EF.
- This fix will be useful, when you write your own EdmGen tool.

Changes for NpgsqlSchema.msl
- I fixed a potential problem.
- This fix will be useful, when you write your own EdmGen tool.