Add bootstrap/node-labels capability to plot_tree

phyloseq 1.3.9
===========
- Added ability to add node labels, bootstrap values to tree graphics
  generated by `plot_tree`.
- The labeling itself is opened-up as a user-provided function to facilitate
  custom node-labeling needs (including symbols and other ggplot2 geoms)
- Commonly-needed functions are provided as newly-documented exported 
  functions in the package:
nodeplotdefault - adds whatever is in the node label to the graphic
nodeplotboot - Adds the labels as bootstrap values, coercing/rounding as needed
nodeplotblank - Ensures that node labels are not added.
- These new functions can be used to give valid arguments to the new
  `nodelabf` argument in `plot_tree`().
- Some other re-organization to `plot_tree` to show more code in 
  the main plot_tree function.