added callbacks to repack_bits and new constellations objects

Hi all,

I added callback functions to both bits per input and output bytes of repack_bits in gr-blocks to make it able to be changed dymatically. 
I've also added two constellations objects in the constellation of gr-digital. One of the constellation object is the natural mapped 8psk and the other is a 16qam. These two constellations are both used in trellis coded modulations based on the paper by G. Ungerboeck "Channel coding with multilevel/phase signals", IEEE Trans. Info Theory, vol. IT-28, pp. 55–67, 1982. 

Please check and merge them to the gnuradio if possible. 

Thanks!
Best,
Zhe