update to version 1.6.0rc1

* added REST proxy client implementing the DHT API
* added optional support for push notifications through the proxy server,
  using the gorush REST API: https://github.com/appleboy/gorush#web-api
* DhtRunner: now able to use DhtProxyClient as a DHT node
* DhtRunner: can switch between local or proxy node during runtime
* SockAddr: added methods to handle IPv6-mapped IPv4 addresses
* iOS: handle SIGPIPE on UDP sockets by recreating sockets
* fixed occasional crash during packet processing