ALLJOYN-394: Fix documentation and unit test for
ProxyBusObject::GetChild.

GetChild will take the relative or absolute path which is the same as
RemoveChild.

Change-Id: Ia4527489c08d80d7d5e6be0a823839e668aaa63b
ALLJOYN-2505: Add missing commands to usage text of sessions.

Change-Id: I38161deddc05574156e92eb50f777b3c94a7b412