v1.3.7 Merge plot_tree updates from Greg Jordan, w/ add rev

phyloseq 1.3.7
===========
- Revisions to plot_tree to improve formatting, organization,
"ladderize"
- contributed by Gregory Jordan
- Color scale option removed from original pull request
- already supported as core ggplot2 functionality through layering
- Added links and roxygen2-header revisions for proper doc formatting
- Fixed bug in which alternative size-variables still labeled as
"abundance" in legend.