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 ("").

This solves [Issue 192](https://github.com/joey711/phyloseq/issues/192), although the minimal version of this bugfix needs to be pushed to release version on Bioconductor as well.