Correct Stream.plot documentation on defaults.
Allow setting tick_format for st.plot(type='relative').
Add a reference time for relative Stream plots.

This can be specified with the 'reftime' argument, so the x-axis will be
relative to it, instead of the start time that's plotted.
Add test images for relative Stream.plot.
Merge pull request #766 from QuLogic/relative-plot-reftime

Reference time for relative plots