data: removed _meta from NDData and children

    - all attributes are now defined in the class, either as simple
      attributes or properties
NDData: added mathematical operators
Spectrum: fixed Quantity operations
TimeSeries: moved spectrogram function into class

    - removed from spectrum.psd module, not useful there
TimeSeries: bug fix for sample_rate==None
Spectrogram: changed initialisation

    - now allows initialisation from another Spectrogram, doesn't copy
      data
Spectrogram: fixed Quantity operations
plotter.ticks: added hack for early matplotlib

    - need to define function if matplotlib doesn't supply it
plotter.ticks: fixed epoch setting in ticks
plotter.Plot: a bunch of little bug fixes
plotter.LayerCollection: added type to colors

    - now recognises AxesImage for coloring
plotter.TimeSeriesPlot: supered add_timeseries

    - now add_timeseries will plot and set epoch
plotter.SpectrogramPlot: moved into own module

    - moved SpectrogramPlot from series.py to spectrogram.py
plotter.SpectrumPlot: moved into own module