Version 0.5 - Refactored network address classes

Version 0.5 refactored the network address classes with an abstract base class and reduced redundant code with templates to create implementations for the different address families.

• (Breaking change) Updated the hierarchy of network address classes, now derived from a common base class.
  • Removed sock_address_ref class. Now a C++ reference to sock_address will replace it (i.e. sock_address&).
  • sock_address is now an abstract base class.
  • All the network address classes now derive from sock_address
  • Consolidates a number of overloaded functions that took different forms of addresses to just take a const sock_address&
  • Adds a new sock_address_any class that can contain any address, and is used by base classes that need a generic address.
• The acceptor and connector classes are still concrete, generic classes, but now a template derives from each of them to specialize.
• The connector and acceptor classes for each address family (tcp_connector, tcp_acceptor, tcp6_connector, etc) are now typedef'ed to template specializations.
• The acceptor::bind() and acceptor::listen() methods are now public.
• CMake build now honors the CMAKE_BUILD_TYPE flag.