Add tell() method to Unpacker

[pohlt]

I store several messages in a single file and use the Unpacker to iterate through the messages.
In my special case I need to know, where in the stream I currently am, so a unpacker.tell() would be super handy.

[jfolz]

See: #227

[pohlt]

Crap, how could I miss that? Works like a charm. Thanks for the pointer, mate!