Remove a few more compiler warnings
Build system tweaks
Remove the relaying behaviour from Mailbox and Post everything
DPP-91 DPP-81 #resolve Add support for child spec "handles"

A new ChildStart constructor allows for returning artitrary state once
a child process has launched, which can be returned to clients of the
supervisor API via the ChildRef.
Cosmetic
DPP-90 #resolve Fixed logging of `unClosure` failures
Merge branch 'development' into resource-pool