Add '__len' to list of overloadable operators.
Add '__len' to list of overloadable operators.

ltjax/luabind-deboostified,alex85k/luabind,mdave88/luabind,rpavlik/luabind,alex85k/luabind,grafi-tt/luabind,MrApplejuice/luabind,plsytj/luabind,Oberon00/luabind,decimad/luabind-deboostified,rpavlik/luabind,rpavlik/luabind,galek/luabind-deboostified,luabind/luabind,rpavlik/luabind,mapbox/luabind,rpavlik/luabind,luabind/luabind,alex85k/luabind,Wohlhabend-Networks/luabind-deboostified,luabind/luabind,lonski/luabind,Oberon00/luabind,luabind/luabind,alex85k/luabind,Kazade/luabind-deboostified,davidar/luabind