Fix complex mask backspace issue

Fix complex mask backspace issue, due to a method swizzling issue in iOS 8.

Ref : https://github.com/blackm00n/AKNumericFormatter/issues/3

Before : 
![aknumericformatter-fail](https://cloud.githubusercontent.com/assets/919691/5493213/a170499a-86b6-11e4-9c85-b622bc462f81.gif)

After : 
![aknumericformatter](https://cloud.githubusercontent.com/assets/919691/5493217/a6fb5fda-86b6-11e4-8e6c-69d7e6ac4169.gif)

Tests were run and succeeded.
I removed a useless `if`. However I think this method do what it have to do, it's just complex. This is just basic stuff that I don't see how I could reduce it more. The complexity is in the `if`s and no action done after requires its own method IMO. But the thing is that we try to *if* the three modes for the formatter. We could only `switch` which mode is *active* and cal a method that do its own loop with no `if` about the current mode. 

How does it sound?