less intrusive memory management

this patch adds the class `memory_cached_mixin` which adds all
member functions and member variables needed by the memory management
subsystem
moved `memory_cached_mixin` to namespace cppa

the mixin is visible in the inheritance diagram for actors, hence it
belongs to namespace cppa rather than cppa::detail
added partial_function::or_else

this feature allows users to define the behavior of an actor by
concatenating partial functions
fixed list of dependencies in documentation