Flat UITextField implementation

Hi,

I wrote a UITextField subclass that exposes some properties that can be used to bring a nice flat appearance to text fields. Please review the code, and if you like it, merge it. To make it easy to test I prepared a FlatUI-like appearance that you can simply drop in your code and test: 

```
[[FUITextField appearance] setFont:[UIFont flatFontOfSize:18.0f]];
[[FUITextField appearance] setBorderStyle:UITextBorderStyleNone];
[[FUITextField appearance] setEdgeInsets:UIEdgeInsetsMake(4, 15, 4, 15)];
[[FUITextField appearance] setTextColor:[UIColor wetAsphaltColor]];
[[FUITextField appearance] setTextFieldColor:[UIColor whiteColor]];
[[FUITextField appearance] setBorderColor:[UIColor turquoiseColor]];
[[FUITextField appearance] setBorderWidth:2.0f];
[[FUITextField appearance] setCornerRadius:6.0f];
```