v1.5.3 plot_heatmap bugfix and revision

bug derives from a change in the NA-handling for labels by ggplot2.
They used to stay in place and be blank. Now NA labels recycle. The
solution was to replace all NA label values with empty character ("").
Merge pull request #194 from joey711/ggplot2-bug

v1.5.3 plot_heatmap bugfix and revision