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

test/http/message.cpp

@@ -22,7 +22,6 @@
 
 #include <beast/detail/unit_test/suite.hpp>
 #include <beast/detail/unit_test/thread.hpp>
-#include <beast/buffers_debug.hpp>
 #include <beast/placeholders.hpp>
 #include <beast/streambuf.hpp>
 #include <beast/http.hpp>
@@ -151,8 +150,7 @@ public:
 
         streambuf rb;
         {
-            request<string_body> req(
-                {beast::http::method_t::http_get, "/", 11});
+            request<string_body> req({"GET", "/", 11});
             req.body = "Beast.HTTP";
             req.headers.replace("Host",
                 ep.address().to_string() + ":" +
@@ -176,7 +174,7 @@ public:
 
     void run() override
     {
-        //testAsio();
+        testAsio();
         pass();
     }
 };