Reduce output cardinality

DBN now outputs 51 nodes instead of 128, for the guitar range of drop C
tuning to max 22 frets instead of full MIDI pitch range.
Final project commit

Multiple independent binary classification for pitch estimation,
L-BFGS-B iterations in fine-tuning DBN training instead of conjugate
gradient. Report pitch recall for low polyphony and high polyphony
frames as well as whether to consider octave errors or not.