`ContextAwareHttpClient` checks whether the http request's Context has
completed/cancelled when the wrapped HttpDoer has completed. If it has,
it returns the Context's err (as opposed to the `error` from
`HttpDoer.Do`), since it's more likely to be useful.

The `HttpDoer` iface was moved to a new `net` package, which is also
where `ContextAwareHttpClient` lives.

The `HttpTokenRetriever` was also updated to use an `HttpDoer` instead
of `FormPoster`. The now-defunct `FormPoster` was removed.

Tests and mocks included.

Fixes #35.