[#8] Network address classes now derived from common base

[fpagliughi]

As described in issue #8:

• Gets rid of sock_address_ref
• Makes sock_address an abstract base class
• Derives all the address classes 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.
• Templates around the generic base classes for connector, acceptor, and stream_socket then individual family versions based on the templates.