add the backward lookup tables for gammadecode,and add  bit_reverse,GammaDecode_B,GetBits_B functions,they do the same thing as GammaDecode_F and GetBits_F, but form the tail(_B denotes backward ,_F denotes forward)
add the backward lookup tables for gammadecode,and add  bit_reverse,GammaDecode_B,GetBits_B functions,they do the same thing as GammaDecode_F and GetBits_F, but form the tail(_B denotes backward ,_F denotes forward)