Addition of gwpy-sounds executable

Created by: dethodav

The executable gwpy-sounds is based on gwpy-plot but produces audio files instead of plots. It takes advantage of the prebuilt infrastructure inside of cliproduct.py with a few additions that are audio specific. Note that as gwpy-sounds uses the exact same methods as gwpy-plot, the two executables could be merged simply by adding exceptions in cliproduct.MakePlot to account for the differences in audio and plot production. Features that were previously introduced to the timeseries class in PR #356 (fshift and wavwrite) are and contained and utilized in this PR.