1.9.3 Massive Revision to plot_tree. Passes tests.

CHANGES IN VERSION 1.9.3
-------------------------

USER-VISIBLE CHANGES

	- Massive Revision to plot_tree()

	- `plot_tree` now uses the `psmelt` function

		- All covariates are available for aesthetic mapping.

	- `plot_tree` substantial speed improvement

		- Uses native ape-package C code for tree computation

		- Efficient `data.table` consolidated graphic data passed to ggplot2

		- Additional arguments: `treetheme` and `justify`

	- `tree_layout` - new, user-accessible function

		- for building alternative trees from phyloseq data.

	- Foundation for solving Issue 313 and Issue 331

		- https://github.com/joey711/phyloseq/issues/313

		- https://github.com/joey711/phyloseq/issues/331