Turbulence: Improve performance by using numexpr
Add zero_crossings

Function that determines at which points zero crossings occur.
phase_spectrum function

Quickly obtain the single-sided phase spectrum.
Use cythonize
Heaviside generator
Noise generators. Use fftw when possible. Normalize as well
Octave: rename keyword order to fraction

It is meant to mean the fraction of the fractional-octave filter, and
not the order of the filter.
Mean square, root mean square and normalise
Rename bandpass filter. Add lowpass, highpass, octavepass and octave_filter
Fixes for renamed bandpass filter
Frequencies and OctaveBand: use fraction, use asarray, extra constructor
Docstring fixes
Merge pull request #87 from FRidh/changes

Changes