[boost] [iterator] Boost zip iterator

First of all thanks for implementing the support of std::pair with the zip iterator in boost 1.60. While testing the feature, I noticed that it is necessary to add the following include #include before code using the zip iterator with std::pair. Just wondering if this was intentional behaviour or if the include should happen directly in zip_iterator.hpp. Anyway, the feature works great and is a real improvement for our projects. Regards Claas