plotter.ticks: fix possible bug in None checking
gwpy: fix custom unit setting for astropy 0.3
TimeSeriesDict: new class for multi-channel data

    - designed to hold multiple channels for a single segment, as an
      analogue to the TimeSeriesList
    - move NDS fetch method to TimeSeriesDict with a simple wrapper in
      the original TimeSeries
io.framecpp: new wrapper to frameCPP

    - method to allow reading multiple channels from a single frame