Make MongooseICE compatible with WebRTC

[rslota]

• 0.0.0.0 IPv4 addr is now used as bind addr for relay socket. Ideally this should be configurable, but it cannot be the same as MONGOOSEICE_UDP_RELAY_IP which was used till now, as IP address of the network interface normally will be different then the actual public IP address that needs to be sent to ICE clients as "relay address". Right now, to just make it work, we'll use wildcard for binding, as this may lead to issues only when setting up multiple MongooseICE instances on one host with multiple network interfaces. When someone actually will need to cover this case, we'll need to simply add another configuration option.
• All responses contain message-integrity attribute if possible. Whenever request is authorized, response will also be properly signed - this is apparently required by most ICE clients.
• Use Distillery 2.1. This fixes only some annoyances while building test releases. Dockerfile has been re-written from scratch to get rid of deprecated mix_docker.

[coveralls]

Coverage decreased (-0.6%) to 89.896% when pulling 5369c28 on fix-relay-bind into 358ba70 on master.

[Neustradamus]

@rslota: Any news about it?