oauth2: export exchanger

Allows users to exchange and refresh tokens without inializing a transport.

config := oauth2.NewConfig(...)
token, err: = config.Exchange(code)

// ...

// refresh token
token, err := config.FetchToken(token)

[jbd, 2014-08-11 17:41:33]

We designed the package auto token refreshes are being made with
oauth2.Transport. Therefore, all interfaces we exposed returns a Transport.

But people who doesn't have anything to do with a RoundTripper still have to
initate one. I'm exposing Exchange, so they can retrieve a token with no use of
https://github.com/golang/oauth2/blob/master/oauth2.go#L152 and use
(*oauth2.Config).FetchToken to manually refresh their tokens.

[bradfitz, 2014-08-11 19:29:53]

LGTM

[jbd, 2014-08-11 19:37:00]

On 2014/08/11 19:29:53, bradfitz wrote:
> LGTM

Submitted as 4c579cbd0d6d2b7377fc15d97e26c189cd7e694f.