[boost] [iterator] Make operators conditionally defined

Hi, I've made this pull request: https://github.com/boostorg/iterator/pull/4 which makes operators defined for iterator_facade conditionally defined depending on the iterator category. As a side effect this change fixes Boost.Graph tests broken by the recent change to boost::next()/prior() [1]. Could someone take a look and merge it? [1] http://lists.boost.org/Archives/boost/2014/06/214788.php