Accept Pathnames and directories as supporting files.

* Accept Pathnames so that you can write something like:

  ```ruby
  LatexToPdf.config.merge!({
    supporting: Rails.root.join("latex.cls") # Rails.root is a Pathname.
  })
  ```
* Do a recursive copy so that you can copy in directories.  In my case, I had a `fonts` directory that I wanted to reference.
* Use `kind_of?` instead of `class == ` to match subclasses.