Version 0.4 - IPv6

Version 0.4 added support for IPv6 and refactored the class hierarchies to better support the different address families without so much redundant code.

• IPv6 support: inet6_address, tcp6_acceptor, tcp_connector, etc.
• (Breaking change) The sock_address class is now contains storage for any type of address and follows copy semantics. Previously it was a non-owning reference class. That reference class now exists as sock_addresss_ref.
• Generic base classses are being re-implemented to use sock_address and sock_address_ref as generic addresses.
• (Breaking change) In the socket class(es) the bool address(address&) and bool peer_address(addr&) forms of getting the socket addresses have been removed in favor of the ones that simply return the address.
  Added get_option() and set_option() methods to the base socketclass.
• The GNU Make build system (Makefile) was deprecated and removed.