Use actor links instead of kill messages.
Merge branch 'master' into topic/linked-actors
Merge branch 'master' into topic/linked-actors

Conflicts:
	src/vast/program.cc
Use additive spawn options.
Merge branch 'master' into topic/linked-actors

Conflicts:
	src/vast/program.cc
	src/vast/program.h
	src/vast/system_monitor.cc
Rename option<T> to optional<T>.

This patch relies on the new overloads for operator-> upstream in libcppa,
as submitted in a pull request Neverlord/libcppa#105.
Merge branch 'master' into topic/linked-actors
Merge branch 'master' into topic/linked-actors
Merge branch 'master' into topic/linked-actors

Conflicts:
	src/vast/program.cc
Link all core component actors together.
Merge branch 'topic/linked-actors'

* topic/linked-actors:
  Link all core component actors together.
  Rename option<T> to optional<T>.
  Use additive spawn options.
  Use actor links instead of kill messages.