v1.3.9 Add node labels, bootstraps to plot_tree()s *unfinished*

Note that there are some bugs popping up still. Needs additional
testing before merging to master branch for general use.
v1.3.9 fixed two bugs

- big bug derived from data availability for adjusting margin in
plot_tree. This follows a reorganization of the code so that margin
adustment can occur on other tree plot methods (right now just
"treeonly")
- Fixed warning from plot_tree() about data missing. This appears to be
needless warning that would scare/distract users. Solved with
na.rm=TRUE param in many geom_*() functions in plot-methods.R
v1.3.9

Updated links in graphics functions to point to the online tutorials at
the top of the in pkg doc
Merge pull request #165 from joey711/fixes

v1.3.9 Add bootstrap/node-labels capability to plot_tree