WebSocket ping, fixes, coverage:

* Improve test coverage
* tests for invokable in composed ops

* Update documentation
* Add License badge to README
* Target Windows 7 SDK and later
* Make role_type private
* Remove extra unused masking functions
* Allow stream reuse / reconnect after failure
* Restructure logic of composed operations
* Allow 0 for read_message_max meaning no limit
* Respect keep alive when building HTTP responses
* Check version in upgrade request
* Response with 426 status on unsupported WebSocket version
* Remove unnecessary Sec-WebSocket-Key in HTTP responses
* Rename to mask_buffer_size

* Remove maybe_throw
* Add ping, async_ping, async_on_pong
* Add ping_op
* Add pong_op
* Fix crash in accept_op
* Fix suspend in close_op
* Fix read_frame_op logic
* Fix crash in read_op
* Fix races in echo sync and async echo servers

include/beast/http/impl/message_v1.ipp

@@ -105,33 +105,6 @@ prepare_content_length(prepare_info& pi,
 
 } // detail
 
-template<bool isRequest, class Body, class Headers>
-void
-prepare_connection(
-    message_v1<isRequest, Body, Headers>& msg)
-{
-    if(msg.version >= 11)
-    {
-        if(! msg.headers.exists("Content-Length") &&
-            ! rfc2616::token_in_list(
-                msg.headers["Transfer-Encoding"], "chunked"))
-            if(! rfc2616::token_in_list(
-                    msg.headers["Connection"], "close"))
-                msg.headers.insert("Connection", "close");
-    }
-    else
-    {
-        if(! msg.headers.exists("Content-Length"))
-        {
-            // VFALCO We are erasing the whole header when we
-            //        should be removing just the keep-alive.
-            if(rfc2616::token_in_list(
-                    msg.headers["Connection"], "keep-alive"))
-                msg.headers.erase("Connection");
-        }
-    }
-}
-
 template<
     bool isRequest, class Body, class Headers,
     class... Options>