C++11 specific code is now an option

We shall now be C++11 aware.
However, C++11 features are, at least for now, optional, _not_ mandatory,
so the code should still be compilable by a non-C++11 capable compiler.
Also note that GNU C++11 extensions are _not_ used (for portability reasons).

As result, binomial_heap supports moving (via rvalue refs), now.