Refactor beast core, http, tests, and examples:

* Fix warnings
* Port cmake scripts to linux
* Add command line options for running test suites
* Add examples to CMakeLists
* Return std::uint64_t from writer::content_length
* basic_parser::write takes asio::const_buffer instead of pointer and size
* Turn message test back on now that it passes
* Rename to http::headers, use std::allocator, remove http_headers
* http::message::method is now a string
* Refactor to_string for ConstBufferSequence
* Remove chunk_encode from the public interface
* Initialize members for default constructed iterators
* Disallow default construction for dependent buffer sequences

Refactor http::message serialization:

* Serialization no longer creates a copy of the
  headers and modifies them
* New function prepare(), sets Connection, Transfer-Encoding,
  Content-Length based on the body attributes and caller options.
  Callers can use prepare() to have the fields set automatically,
  or they can set the fields manually.
* Use write for operator<<
* Tests for serialization

doc/beast.qbk

@@ -115,9 +115,10 @@ int main()
     using namespace beast::http;
 
     // Send HTTP request using beast
-    request<empty_body> req({method_t::http_get, "/", 11});
+    request<empty_body> req({"GET", "/", 11});
     req.headers.replace("Host", host + ":" + std::to_string(sock.remote_endpoint().port()));
     req.headers.replace("User-Agent", "Beast");
+    prepare(req);
     write(sock, req);
 
     // Receive and print HTTP response using beast
@@ -130,8 +131,8 @@ int main()
 
 Establish a WebSocket connection, send a message and receive the reply:
 ```
+#include <beast/to_string.hpp>
 #include <beast/websocket.hpp>
-#include <beast/buffers_debug.hpp>
 #include <boost/asio.hpp>
 #include <iostream>
 #include <string>
@@ -158,8 +159,7 @@ int main()
     opcode op;
     ws.read(op, sb);
     ws.close(close_code::normal);
-    std::cout <<
-        beast::debug::buffers_to_string(sb.data()) << "\n";
+    std::cout << to_string(sb.data()) << "\n";
 }
 ```