Add pointers to some speech recognition papers.

Including:

 - A good tutorial on hidden Markov models.

 - A good reference on weighted FSTs which are used to compose the (context-dependent) phoneme-level acoustic model, pronunciation lexicon, and n-gram language model to generate decoding graphs in modern large vocabulary recognition systems.

 - A cool paper that demonstrates how the Viterbi algorithm used for recognition can be generalized to search over different segmentations of the input spectrogram -- simultaneously doing source separation *and* speech recognition.