TimeSeries.psd: minor type casting fix
SpectrumAxes: new plot axes

   - designed for display of amplitude/power versus frequency
Series: bug fix in metadata ownership when slicing
Series: improved index property
Array2D: improved index_x, index_y properties
Array2D: now return span_x, span_y as Segment
TimeSeriesAxes: added plot_spectrogram

    - pcolormesh spectrogram plotter, handles logarithmic scales nicely
fix whitespace errors
auto_refresh: now returns function output
auto_refresh: fix corner case of attribute get
Plot: new add_colorbar method

    - generates a new colorbar axes on the current plot, mapping against
      a collection (mappable) and its axes
TimeSeriesAxes: added auto_refreshing to methods