Update paths to use `pion` instead of `gortc`

[Sean-Der]

I think the big ones are

• Move appveyor
• Update API compatibility checks
• (Maybe move Dockerfiles?)
• (Maybe move demo server to pion.ly?)

seaduboi@38f9d359441f:~/go/src/github.com/pion/stun$ rg gortc
AUTHORS
8:The gortc project

Makefile
46: go get gortc.io/api
49: docker build -t gortc/stun .

stun.go
10:// See examples for Message for basic usage, or https://github.com/gortc/turn

message_test.go
828:        NewSoftware("gortc/stun"),
849:        NewSoftware("gortc/stun"),
881:        NewSoftware("gortc/stun"),

appveyor.yml
11:clone_folder: c:\gopath\src\github.com\gortc\stun

README.md
111:  * Explicit API backward compatibility [check](https://github.com/gortc/api), see `api` directory
113:See [TeamCity project](https://tc.gortc.io/project.html?projectId=stun&guest=1) and `e2e` directory

e2e/docker-compose.yml
15:    image: gortc/coturn

e2e/test.sh
32:docker build -t gortc/tcpdump -f tcpdump.Dockerfile .
42:    --name ci_stun-tcpdump --net=host gortc/tcpdump

cmd/stun-traversal/Readme.md
1:stun-traversal is a small NAT traversal proof of concept using package gortc/stun. Peer exchange is done manually using stdin.

cmd/stun-traversal/main.go
17: server = flag.String("server", fmt.Sprintf("gortc.io:3478"), "Stun server address")

[ernado]

I think that was already done a while ago.