Fixes related to CSV exporter:
  - CSV headers include data names, if available
  - Exporter correctly handles items with no data
  - pg.plot() avoids creating empty data item
  - removed call to reduce() from exporter; not available in python 3
  - Gave .name() methods to PlotDataItem, PlotCurveItem, and ScatterPlotItem
Fix examples/hdf5.py to work properly with --test