Updated String library to use C++11 iterators.

This will allow using the String library in a ranged for loop:

```C++
void setup() {
  
  Serial.begin( 9600 );
  String s = "Hi, this is a test of C++11 iterators.";
  
  for( char c : s ){
    Serial.print( "Character in string: " );
    Serial.println( c );
  }
}
void loop(){}
```

This is placed inside the check for C++11 which is also due for update here: https://github.com/matthijskooijman/Arduino/commit/6624d614fa1f95c26c471ae494012923e03a2e78