option => optional
option => optional

this patch renames `optional` to `optional` in advance of porting
libcppa to boost

actor-framework/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework