Dropped events, anonymous error pages

A new kind of event, "dropped" can be used to track when an outgoing message has been dropped due to current backpressure settings. This is useful in cases where tracking the status is otherwise impossible, such as when using pub/sub. See example "EchoServer.cpp" for usage.

Another change is the removal of graphical error pages (the HTML body is gone) when compiling with UWS_HTTPRESPONSE_NO_WRITEMARK.