Smoothes and slightly tightens Intro to RNeXML leader.
Adds reference for TreeBASE.
Edits for Reading Phylogenetics Trees section.
Edits for Reading Character Data.

Less extensive edits here, mostly to tighten text and improve flow.
Edits the Writing NeXML section.

Most of these are fairly minor. I did separate the explanation for creating
`nexml` objects into its own subsection, for better consistency with the
part about reading NeXML files.
Merge pull request #101 from ropensci/hl-intro-rnexml-edits

Various edits to the Intro to RNeXML section