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.
Use older flag to enable C++11.
Use the range-based for loop of C++11 (not finished yet).
Remove unused fields.
More range-based for.
Merge branch 'cpp11' of https://github.com/christopho/solarus into point_and_size
Made Rectangle's one-parameter constructors explicit.