Refactor beast::asio:
* New tools for completion handlers:
  - make_wrapped provides composed io_service execution guarantees.
    Executes bind() functors in the same context as a handler,
    with an option for setting asio_handler_is_continuation.
  - shared_handler type-erases any completion handler.
  - buffer_sequence type-erases templated BufferSequences
  - abstract_socket replaces Socket
  - socket_wrapper replaces SocketWrapper
  - beast::asio placeholders to work with std::bind

* Removed obsolete classes and functions
  - AbstractHandler
  - ComposedAsyncOperation
  - SharedFunction
  - SharedHandler
  - SharedHandlerAllocator
  - SharedHandlerPtr
  - SharedHandlerType
  - SocketBase
  - SocketWrapperStrand
  - wrapHandler

* Refactored classes to use new tools
  - abstract_socket
  - socket_wrapper
  - HandshakeDetector
  - HttpClientType

* Miscellanous tidying
  - socket classes moved to beast::asio namespace
  - beast asio files provide their own namespace declaration.
  - Fix IsCallPossible conflicting template parameter name
  - Use <boost/get_pointer.hpp> for C++11 compatibility.
  - Remove extraneous include path from build environment.