Added string iterator in sam core.

See #2179

Author: cmaglie

cores/arduino/WString.h

@@ -161,6 +161,8 @@ class String
 	void toCharArray(char *buf, unsigned int bufsize, unsigned int index=0) const
 		{getBytes((unsigned char *)buf, bufsize, index);}
 	const char * c_str() const { return buffer; }
+	const char* begin() { return c_str(); }
+	const char* end() { return c_str() + length(); }	
 
 	// search
 	int indexOf( char ch ) const;