Make NetworkBuffer a move only class.

NetworkBuffer objects cannot be copied after creation, but they can be moved
as a rvalue reference from temporary NetworkBuffer objects.

e.g. A NetworkBuffer created in a function and returned by value will be moved
(as a rvalue) to the receiving (lvalue) NetworkBuffer instead of being copied.
Merge pull request #22 from tcoyne/move-only-buffer

Fix #20 Make NetworkBuffer a move only class.