add hash_combine (from Boost) and hashing for std::pair's
add hash_combine (from Boost) and hashing for std::pair's

--HG--
branch : cxx11

florianhauer/ompl,davetcoleman/ompl,davetcoleman/ompl,davetcoleman/ompl,jvgomez/ompl,jvgomez/ompl,florianhauer/ompl,utiasASRL/batch-informed-trees,utiasASRL/batch-informed-trees,jvgomez/ompl,jvgomez/ompl,utiasASRL/batch-informed-trees,florianhauer/ompl,jvgomez/ompl,florianhauer/ompl,florianhauer/ompl,florianhauer/ompl,davetcoleman/ompl,utiasASRL/batch-informed-trees,davetcoleman/ompl,utiasASRL/batch-informed-trees,utiasASRL/batch-informed-trees,jvgomez/ompl,davetcoleman/ompl