Added reset_burn_in

This does what  #128 suggests and adds a ``reset_burn_in`` method.  So I think this closes #128

@dfm, I confess I don't understand why this works: I'm just reproducing the "old" `reset` prior to #124, but I don't fully understand why setting `iterations` and `naccepted` to 0 resets the chain... But I assume that's the right way since it worked before? 

cc @ipashchenko