pheatmap class with grid.draw and print methods

[GuangchuangYu]

re-PR, #42.

• add pheatmap class attribute
• define grid.draw method, so that ggplot2::ggsave can be used to export pheatmap object
• define print method, so that the heatmap will be drawn interactively

To make it simple for you to read the diff, I didn't update roxygen (namespace for grid and print method).

[raivokolde]

Thanks for the additions!