Optionally provide a custom http.Client

[nealey]

I added this so I could get out of Google Compute Engine, but this could also be helpful for people behind proxies or using other unusual configurations. This simply adds a new auth.NewClientWithHttpClient method, everything else remains the same.

[andig]

Should probably not accept a client struct but rather an interface to allow testability.

[andig]

We've implemented this in https://github.com/bogosj/tesla