v0.1.19 Bug fix release

Functional changes:

• Removed margin argument from dendro_data.rpart, since this had no effect on plot.

Bug fixes:

• Fixed issue #20: Plot data is dependent on device, causing errors and spurious new devices
• Fixed issue #24: Labels don't print in ggdendrogram() with package scales_0.4
• Fixed issue #22: Margin not working for dendro_data.rpart