v1.3.13 Fix Issue 166, plot_scree warnings during pkg build

phyloseq 1.3.13
===========
- Fixed unofficial warning from missing (unregistered) classes
  of ordination objects. Stems from the internal get_eigenvalue
  being defined as S4 generic instead of S3.
Solves the following logged issue:
https://github.com/joey711/phyloseq/issues/166

- Name of get_eigenvalue generic replaced with extract_eigenvalue,
  in addition to re-defining as S3 generic.

- Fixed ggplot2 warning from over-specified "bins" in the
  plot_scree bar plot. Changed to stat="identity", solves problem

- Fixed warning when eigenvalues get (slightly) negative. They
  are now set to 0.0 for the purposes of plotting in plot_scree.