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.
v1.3.14 Reduced compute time for in-doc examples

phyloseq 1.3.14
===========
- Shortened/replaced examples in the longest-running doc examples.
  This substantially reduces the time it takes to run package checks.

- Focused on worst offenders

- Created a new online tutorial for `subset_ord_plot`.
  Linked to it from `plot_ordination`

- `plot_taxa_bar` moved to deprecated function file where it belongs,
   and examples completely removed from doc.
Merge pull request #175 from joey711/fixes

v1.3.14 Fix plot_scree warning during pkg build; reduce compute time for in-doc examples