Heatmap added x-ticks based on the time points, not absolute time.

When a DataFrame is passed into matplotlib.imshow, the Axes no longer
cares about the column names.  We want the plot to take into account the
sampling period AND the unit scaling if the user specifies one.

I copied over the method I implemented for boxplot.py -- it essentially
'tricks' matplotlib by redefining the labels rather than change the
time-axis values (see
https://stackoverflow.com/questions/10171618/changing-plot-scale-by-a-factor-in-matplotlib).
matplotlib's xscale does not support simple linear re-scaling; in any
case, having time-axis values does not make sense for imshow.

This commit addresses issue #20.