Add new configre arguments
Remove unnecessary atomic load operation
Add lazy_init spawn option
Fix memory leak with become(), close #206
Improve documentation of IO system messages
Add either/or interface definitions

The new `replies_to<...>::with_either<...>::or_else<...>` notation allows
developers to have success/failure messages for typed actors. Message handlers
need to use the corresponding `either<...>::or_else<...>` result type.