v1.3.7 Improvements to plot_tree formatting

phyloseq 1.3.7
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- 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.