Ability to switch what Http Client is used. 

This let's you change what the http client is used, but defaults to the `http.DefaultClient`.

This is particularly useful on Google App Engine, where http.DefaultClient isn't available, and you can't use it. You *have* to use their [urlfetch](https://developers.google.com/appengine/docs/go/urlfetch/) service.

This was directly inspired by how [SendGrid's integration](https://github.com/sendgrid/sendgrid-go#appengine-example) does it.