PDF Writer: Attempts to convert images to pdf renderable formats

Now depends on the JuicyPixels library.

Will attempt to convert an image (gif, tiff, bmp) to png when converting
to pdf.
LaTeX Writer: Added missing closing braces to hyperdef commands
Merge pull request #1527 from mpickering/juicypixels

Attempts to convert gif, tiff and bmp to png in pdf writer