API break to rename functions based on feedback.

A key part of this branch is to provide documentation.  The GoDoc tool treats a doc.go file specially, and it seemed like a reasonable place to have the package documentation.

The arrar.go file now just contains the publicly exported functions from the package, with the other parts broken into individual files.

Key changes:
  arrar.New -> arrar.Errorf
  arrar.Translate -> arrar.Wrap

The non-format Annotate function was removed, a package level LastError function is added, and a DefaultErrorStack added.