Refactor Overlay for Structured Network support:
* Move overlay up one directory
* Add abstract_protocol_handler, message_stream
* Add peer_protocol_detector
* Tidy up some declarations
* Use strand::running_in_this_thread instead of bool
* Update README.md
* Replace protocol message read loop:
  - Process data in arbitrary size chunks
  - message_stream extracts individual messages
  - peer_protocol_detector identifies the handshake
  - abstract_protocol_handler used for dispatching messages
* Remove unused protocol message types:
  - mtACCOUNT
  - mtCONTACT
  - mtERROR
  - mtGET_ACCOUNT
  - mtGET_CONTACTS
  - mtGET_VALIDATIONS
  - mtSEARCH_TRANSACTION
  - mtUNUSED_FIELD

Conflicts:
	src/ripple/module/app/main/Application.cpp
	src/ripple/module/app/misc/NetworkOPs.cpp
	src/ripple/module/app/peers/PeerSet.cpp