Support for webhooks and getting current user info

I added the following methods for dealing with webhooks:
* `registerWebhook()`
* `listWebhooks()`
* `getWebhook()`
* `deleteWebhook()`

Not related to webhooks, but this method returns information on the currently authenticated user:
* `getCurrentUser()`

To get the above working, I had to extend `makeUri()` to take `altApiVersion` as an argument so that you can use an `altBase` that has a different version.