Updated String library to use C++11 iterators.

This will allow using the String library in a ranged for loop:

```C++
String s = "Hi, this is a test";

for( char c : s )
  Serial.print( c );
```

Author: Chris--A

Diff for: hardware/arduino/avr/cores/arduino/WString.h

@@ -89,6 +89,8 @@ class String
        #if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
 	String & operator = (String &&rval);
 	String & operator = (StringSumHelper &&rval);
+	auto begin() -> const char* { return c_str(); }
+	auto end() -> const char* { return c_str() + length(); }	
 	#endif
 
 	// concatenate (works w/ built-in types)