Experimental: compile with C++11.
Replace NULL by nullptr.
Merge branch 'master' into cpp11

Conflicts:
	include/entities/CustomEntity.h
Use the auto keyword do avoid verbose iterator declarations.
Another C++11 progress.
More auto.
Rectangle constructor is explicit only for 1 parameter.
Rule of zero for Rectangle copy constructor and destructor.
Adding override qualifiers.
Used list initialization syntax for return.
Merge remote-tracking branch 'upstream/cpp11' into cpp11
Simplified Rectangle construction.
Replaced push functions by emplace functions.
Removed useless const qualifications for integer return types.
Left only two meaningful constructors with two and four parameters.
Merge pull request #580 from Morwenn/cpp11

Revamp Rectangle construction, use emplace functions, clean return types
Used std::atan2 to handle most of the work in get_distance.
Removed redundant flush instructions.
Merge pull request #582 from Morwenn/get_distance

Used std::atan2 to handle most of the work in get_distance.
Merge pull request #583 from Morwenn/remove_flush

Removed redundant flush instructions.