Add support for Cli_ReadMultiVars

This change add supports for the Cli_ReadMultiVars function.  I have also included an example.

actually it looks quite good all, but one question. Why do you build up the data_items this way? cant you just make a python list of 3 data_items with
```
data_items = S7DataItem(c_int32(S7AreaDB), c_int32(S7WLByte), c_int32(0) ... etc.)
```
this is what I ment with C stylish coding, at least it looks like that to me.