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/websocket/impl/write_op.ipp

@@ -109,7 +109,7 @@ operator()(error_code ec, bool again)
 {
     auto& d = *d_;
     d.cont = d.cont || again;
-    while(! ec && d.state != 99)
+    if(! ec)
     {
         switch(d.state)
         {
@@ -126,6 +126,9 @@ operator()(error_code ec, bool again)
             d.ws.async_write_frame(fin, pb, std::move(*this));
             return;
         }
+
+        case 99:
+            break;
         }
     }
     d.h(ec);