Support for C++11 range-based for

[fabianh]

As

for(NodeIt n(g);n!=INVALID;++n)

is a very common construct, it would be convenient to have access to the new range-based for loop, which turns the above into the very natural

for(Node n: g)

I don't have a good understanding of the library structure, so the solution I am proposing is not very generic (see attached file).

[Alpar Juttner]

Feature is actually under development, see #325