Add Flags template

Flags is a typecons template for storing OR combinaton of enum values in a type safe fashion.

I think that the possibility to use bitwise operation on enums is a bit of a hole in D type system;
Flags provides a type safe alternative modeled after Qt's Q_FLAGS
> I think he meant @safe pure nothrow: as the first line in the body of the struct. 

Oh yeah, they're attributes like the access control ones after all.