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

include/beast/impl/basic_streambuf.ipp

@@ -118,7 +118,7 @@ public:
 template<class Allocator>
 class basic_streambuf<Allocator>::const_buffers_type
 {
-    basic_streambuf const* sb_ = nullptr;
+    basic_streambuf const* sb_;
 
     friend class basic_streambuf;
 
@@ -126,12 +126,12 @@ class basic_streambuf<Allocator>::const_buffers_type
     const_buffers_type(basic_streambuf const& sb);
 
 public:
-    /// Why?
+    // Why?
     using value_type = boost::asio::const_buffer;
 
     class const_iterator;
 
-    const_buffers_type() = default;
+    const_buffers_type() = delete;
     const_buffers_type(const_buffers_type const&) = default;
     const_buffers_type& operator=(const_buffers_type const&) = default;
 
@@ -157,7 +157,7 @@ public:
 
     class const_iterator;
 
-    mutable_buffers_type() = default;
+    mutable_buffers_type() = delete;
     mutable_buffers_type(mutable_buffers_type const&) = default;
     mutable_buffers_type& operator=(mutable_buffers_type const&) = default;