Implement the missing getElementById and getElementsById methods

At some point in time, appium's iOS controller was changed to call the getElementById function on `au` when finding an element via the 'id' strategy. That function doesn't exist. This pull request implements both getElementById and getElementsById. All I really did was copy the old implementation in the iOS controller into the new functions.

The one change that I did make while testing this was to remove 'value' from the fuzzy match predicate. I kept getting an exception from UIAutomation because one of the elements in the TestApp has a value of 0 and the predicate match was choking on it. I personally think fuzzy searching the name and label should be more than sufficient.