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;