make createConnection overridable by sub classes

We're working on an Authy integration and would like to get the client running on App Engine. Currently we get this exception:
```
java.lang.ClassCastException: com.google.apphosting.utils.security.urlfetch.URLFetchServiceStreamHandler$Connection cannot be cast to javax.net.ssl.HttpsURLConnection
	at com.authy.api.Resource.createConnection(Unknown Source)
	at com.authy.api.Resource.request(Unknown Source)
	at com.authy.api.Resource.post(Unknown Source)
	at com.authy.api.Users.createUser(Unknown Source)
```

App Engine supports HttpURLConnection, but not HttpsURLConnection.
So we need to be able to easily override this behaviour, hence this change.

Please merge and crate a new Jar.