Disbursements

This pull request implements disbursement webhooks and disbursement details on transactions.

There are a few gotchas here I'd like feedback on.  (I'm also very open to feedback on the rest - I'm a "baby gopher" as the phrase apparently goes.)

* Disbursement details by nature of the sandbox are seemingly not possible to test in a merchant-account-agnostic way. I wrote a test that passed using an old transaction from my sandbox, and have swapped out the values here for a transaction on the account the CI server points to. It's noted in `TESTING.md`.

* Disbursements rely on search for returning the details of the transactions in that disbursement. For testing this method, I used the id from the testing account transaction from the previous bullet point. The id can be replicated easily by others in their sandbox; again the details are in `TESTING.md`.

* `Disbursement` cannot be both the name of a type and a const, so I made the webhook kind `DisbursementWebhook`. For consistency's sake, I also changed the rest of the webhook kinds. That makes this pull request backwards incompatible.  It also pulls it out of parallel with the rest of the client libraries.

* All fields in `DisbursementDetails` are of type `String`, even though half should be `float64`s or `bool`s. This is to prevent errors when unmarshaling nil values. [There is precedent](https://github.com/lionelbarrow/braintree-go/blob/master/plan.go#L23-L27) for doing it this way; [there is also a second workaround buried in here](https://github.com/lionelbarrow/braintree-go/commit/d3b58b43d50fb42c53eb0a005abaf6cbb3fe9397) which doesn't strike me as optimal because it prevents you from getting the value back out of the API. The underlying cause is [an issue that was accepted in the language](https://code.google.com/p/go/issues/detail?id=5857) thanks to @eaigner. Since that issue was accepted, Go 1.2 was released, which includes an xml `Unmarshaler` interface. This may be the right thing to use short of writing a custom XML parser, but then use of this library is restricted a version of Go that's only been around for about four months. What are your feelings about this?