Add Google OAuth2 client authentication

[vitorarins]

[fernandoguedes]

The golang provides a lib to authetication using oauth2: https://godoc.org/golang.org/x/oauth2

[i4ki]

AFAIK this lib is client-only. There's other libraries supporting client & server in Go, but not official ones.

[vitorarins]

Oh, I will update the issue title. We are actually looking for Google's OAuth authentication, not just OAuth.

[i4ki]

google oauth uses oauth2, no?

[vitorarins]

Yess! Thanks for pointing that out 😉