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/core/prepare_buffers.cpp

@@ -89,6 +89,7 @@ public:
 
     void testIterator()
     {
+        using boost::asio::buffer_size;
         using boost::asio::const_buffer;
         char b[3];
         std::array<const_buffer, 3> bs{{
@@ -98,7 +99,12 @@ public:
         auto pb = prepare_buffers(2, bs);
         std::size_t n = 0;
         for(auto it = pb.end(); it != pb.begin(); --it)
+        {
+            decltype(pb)::const_iterator it2(std::move(it));
+            expect(buffer_size(*it2) == 1);
+            it = std::move(it2);
             ++n;
+        }
         expect(n == 2);
     }