JCLOUDS-43. add scoped transaction support to ultradns-ws

Adds support to batch write operations into the same server-side transaction.

https://issues.apache.org/jira/browse/JCLOUDS-43

Here's the expected usage.

```
String txId = ultraDNSApi.getTransactionApi().start();
try {
   // perform operations
   ultraDNSApi.getTransactionApi().commit(txId);
} catch (Throwable t) {
   ultraDNSApi.getTransactionApi().rollback(txId);
   throw propagate(t);
}
```