Allow to add and remove handlers at runtime.

A GCDWebServerHandler reference is returned by the addHandler method.
The caller can use it later to remove the handler.
Added missing locking. Changes of handlers at runtime should be safe now.

I'm not entirely happy with requestAndHandler: forMethod: url: headers: path: query: but the locking must happen inside GCDWebServer.
introduced dataBlock to allow handling of large files / buffers
Fixed handling of multiple data chunks. Respect the maxLenth parameter.
Evaluate the error property and stop if it is set.
Allow the preBlock to add headers.
Merge branch 'DataBlockResponseRe2' into development
Merge branch 'LOG-Renames' into development