fix stdio "cannot listen", fix udp goroutine leak

fix multiple getters panic

added client tls skip-verify and ca flags (closes jpillora#129 closes j…

…pillora#116)

thanks to @mvladev and @alfonso-presa, I couldn't merge your PRs since I've done a massive refactor, though I borrowed snippets from them

release android binaries (closes jpillora#147)

add tls (and auto-certificate) support

add ResetUsers server method (closes jpillora#175)

merge client stdio support (closes jpillora#166 thanks @BoleynSu!)

minor doc changes, test docker build

doc changes, fixed docker auto-build, moved licence to standalone file

merge client reverse+socks PR by @aus (closes jpillora#78)

- @aus's branch also includes PR for
custom headers, thanks @AkeemMcLennon  (closes jpillora#90)
- support client connections via socks
- add more architectures to the built releases
- remove vendor (go.sum will enforce correct deps)