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.
Not related with this pull request but we should clean `formatString` method. This method have too many lines with a high complexity. We should reduce each root condition in the first `if` (inside the loop) with small methods. This is my personal opinion but what do you think?