Develop

## New modules
* ``librosa.segment`` functions for structural segmentation
* ``librosa.onset`` onset strength and detection
* ``librosa.display`` helper functions for spectrogram visualization
* ``librosa.filters`` functions to generate filter banks
* ``librosa.decompose`` matrix factorization and harmonic-percussive separation

## New features
* helper function for magnitude/phase separation ``librosa.magphase()``
* ``librosa.load()`` now supports offset and truncation
* ``librosa.peak_pick()`` to select peaks from a time series
* ``librosa.phase_vocoder()`` for time-stretching spectrogram
* ``librosa.cqt()`` constant-q transform
* ``librosa.ifgram`` instantaneous frequency spectrogram
* Standardized frequency conversion routines
* ``librosa.feature.ifptrack`` instantaneous frequency pitch tracking

Plus numerous bug fixes and improved documentation.