Add more documentation/examples for extracting content from PDFs

[michaelrsweet]

As mentioned in issue #92, the current documentation for pdfioStreamGetToken doesn't explain that whitespace and comments are skipped, name values start with '/', nor that PDF operators and values are returned as strings starting with a letter.

It would be useful to expand the pdf2text.c example code discussion as well, and maybe add a pdf2images.c example as another way to show how to extract information from a PDF file.