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

examples/http_crawl.cpp

@@ -17,8 +17,8 @@
 */
 //==============================================================================
 
-#include "http_stream.h"
-#include "urls_large_data.h"
+#include "http_stream.hpp"
+#include "urls_large_data.hpp"
 
 #include <boost/asio.hpp>
 #include <iostream>
@@ -46,10 +46,11 @@ int main(int, char const*[])
             stream<ip::tcp::socket> hs(ios);
             connect(hs.lowest_layer(), it);
             auto ep = hs.lowest_layer().remote_endpoint();
-            request<empty_body> req({method_t::http_get, "/", 11});
+            request<empty_body> req({"GET", "/", 11});
             req.headers.insert("Host", host +
                 std::string(":") + std::to_string(ep.port()));
             req.headers.insert("User-Agent", "beast/http");
+            prepare(req);
             hs.write(req);
             response<string_body> resp;
             hs.read(resp);