API for dynamically adding and removing childrenspec to the supervisor

[leonlee2013]

Similar to these Erlang APIs：#start_child-2,#terminate_child-2,#delete_child-2
Here is an attempt to write some code to illustrate some of my idea：leonlee2013@47f49ae

[halturin]

This feature will be introduced in the next release 3.0. Working on it now.