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

test/websocket/detail/mask.cpp

@@ -5,7 +5,9 @@
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
+// Test that header file is self-contained.
 #include <beast/websocket/detail/mask.hpp>
+
 #include <beast/unit_test/suite.hpp>
 
 namespace beast {