v1.3.8 fix bug in plot_tree if sample names start with number
v1.3.8 updated NEWS as well

- Fix bug in plot_tree graphic if sample names start with a number
- This fixes https://github.com/joey711/phyloseq/issues/149
v1.3.8 added default fill argument to point layer

- Also added fill argument to default aesthetic map definition, useful
if fillable shapes defined in subsequent layers
Merge pull request #163 from joey711/fixes

v1.3.8 merged from fixes branch. Solves two bugs, including issue 149.