Add [overlay] configuration section

Conflicts:
	src/ripple/overlay/impl/OverlayImpl.cpp
	src/ripple/overlay/impl/OverlayImpl.h
	src/ripple/overlay/impl/PeerImp.cpp
	src/ripple/overlay/impl/PeerImp.h
Always detect ssl in HTTP server
Add new Handler interfaces
Persist HTTP peer io_service::work lifetime
Add HTTP::Writer interface:
The Writer interface allows clients to supply a "pull" style object to provide
responses to HTTP Requests. The interface is designed to support a coroutine
based implementation.
Tidy up includes
Split implementation into three headers
Use ssl_bundle in SSLPeer
Call HTTP Handler with legacy peer handshake
Use ssl_bundle in Overlay::Peer
Construct Overlay::Peer with ssl_bundle and legacy handshake
Pass legacy peer handshakes through Overlay
Add doors section in cfg