Add OAuth2 HTTP handler

[kgilpin]

Adding support for OAuth2 handler will enable us to support all these backends : https://github.com/golang/oauth2

The handler should simply perform:

token := oauth2.Token{AccessToken: string(token)}
token.SetAuthHeader(r)

[izgeri]

Note: this could potentially be built using the generic HTTP connector or possibly even written via config.