rac_textSignal should accommodate 'clear when editing begins'

A question came up recently on StackOverflow asking about "clear when editing begins". You can see that [here](http://stackoverflow.com/questions/21616418/reactivecocoa-observing-isfirstresponder-property-and-uitextfield-with-clearso/21619091?noredirect=1#comment32697282_21619091).

User @allprog mentioned that this has already been discussed in Issue [516](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/516).

However, 516 seems to address programmatic updates. This PR simply adds another valid control event to the mix.

Currently, the problem is that when a UITextField has "clear when editing begins" selected in IB (or `clearsOnBeginEditing` in code), the `-rac_textSignal` won't send along the text value to reflect that change.  So when a user starts editing a field and that field clears, it's more cumbersome for the developer to accommodate that in order to, say, update the `enabled` status of another control.

I tested this change by logging the updated values through `-combineLatest:reduceEach:`, and found that the text value is indeed empty by the time that `UIControlEventEditingDidBegin` is fired.