add handler base for use by client and server

Adds the handler base class, client::handler, and server::handler
so a connection doesn't need to query itself to know what type of
connection it is.
replace list of pairs with map for message header
remove unused active_handler
add templated add_to_header for numeric values
send method string rather than integer in prologue
send client request in terms of sinks and sources

This is in contrast to the old way of sending which used WinHTTP
for client communication.
improve templated add_to_headers
basic http::client sends requests without WinHTTP

This commit also parses responses properly and forwards callbacks
to the appropriate handles. Only message_body_was_received will
callback to user code at the moment (response_f).
Merge branch 'feature/platform-independent-client'