PDF generation improvements.

* `Text.Pandoc.PDF` exports `makePDF` instead of `tex2pdf`.
  (API change.)
* `makePDF` walks the pandoc AST and checks for the existence of
  images in the local directory.  If they are not found, it attempts
  to find them, either in the directory containing the first source
  file, or at an absolute URL, or at a URL relative to the base URL
  of the first command line argument.
* Closes #917.