Don't pollute the library's namespace with the whole std::

As a library writer, one should refrain from `using namespace std;` in the headers. This will flood the library's namespace with all definitions from `std::` and facilitate name conflicts in **any program** doing `#include <wb.h>`.

We may hope, that *probably* this will not cause any problems in the scope of the submission system; but it's still a [poor engineering practice][*] and will inadvertently *teach* bad habits to the users of the library — for no good reason.

[*]: http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice