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
Minor nitpick: please don't use Egyptian braces. See the [Phobos style guide] (http://dlang.org/dstyle.html).