Refactor beast::asio:
* Add beast::asio::placeholders for use with std::bind
* New tools for completion handlers:
  - shared_handler type-erases any completion handler.
  - make_wrapped provides composed io_service execution guarantees.
* Removed obsolete classes and functions
  - AbstractHandler
  - ComposedAsyncOperation
  - SharedFunction
  - SharedHandler
  - SharedHandlerAllocator
  - SharedHandlerPtr
  - SharedHandlerType
  - SocketBase
  - SocketWrapperStrand
  - wrapHandler
* Refactored classes to use new tools
  - HandshakeDetector
  - HttpClientType
  - Socket
  - SocketWrapper
* Interface for propagating asio_handler_is_continuation
* Tidy up IsCallPossible() conflicting template parameter name
* beast asio files provide their own namespace declaration.
* Use <boost/get_pointer.hpp> for C++11 compatibility.
* Remove extraneous include path from build environment.