update of backward_bigram_lattice, so that backward_bigram_lattice[i][j][k] = bigram represented as the tuple (sent[k:j], sent[j:i]) if j in backward_bigram_lattice[i] and k in backward_bigram_lattice[i][j]

It is for fast retrieval of all the possible bigrams that ends at position i of the sentence.
commit tiny change in search function
Merge pull request #1 from biye2014/data_struct_update_lattice_gen

Data struct update lattice gen