Added optional async to Autocomplete Datasource

[kav]

Also updated the example and created a manager
to demonstrate the pattern.

[sibljon]

Thank you for the contribution, but an exception is raised when scrolling in the example project. The problem is that -[NSString hasPrefix:] is begin passed nil as an argument. Is there a way we can prevent this from happening?

[sibljon]

Here's the exception I'm seeing:

[kav]

Weird. Can't seem to reproduce. Did you have anything entered or UI focused?
I can guard against the nil but I'd like to dig into why it's happening.

[sibljon]

It happens to me when I scroll down, the back up, without having entered any text in the text fields.

It seems to be that the [UITextField resignFirstResponder] message is being sent while the text is nil. This is causing stringToLookFor to be nil in [stringToCompare hasPrefix:stringToLookFor].

I think guarding against stringToLookFor being nil is a good call, such that the return completionHandler(@""); line gets called in HTAsyncAutocompleteManager immediately thereafter.

Here's a screenshot that shows the stack trace: