Add "dontprint this page" functionality. Works but still very little user feedback.

* If current tab shows a page that represents a document that can be
  dontprinted, a dontprint icon appears in the address bar. Clicking
  that icon initiates a download, conversion, email to the Kindle.
* Dontprint uses functionality originally developed for Zotero to
  detect web pages that represent scientific articles, to extract
  meta data from the website and to download the PDF.
* Other change: preference string
  "extensions.zotero.dontprint.outputdirectory"
  is no longer used. Output from k2pdfopt is now written to a
  temporary directory and deleted after uploading.
* TODO: There's no user feedback except for a confirmation tab
  when everything is done.
* TODO: Error handling; especially, empty downloaded PDFs are not
  recognized as errors. This happens, e.g., when the content
  provider throws a captcha at the client, as on APS sites.