add pdf media type

add pdf media type

The new media type supports pdf and a subset of media recognized by libreoffice via
unoconv.

Every document added goes through:
* conversion to pdf with unoconv if not already a pdf
* creation of thumbnail and medium sized image, and pdfinfo generates
 some information (even for unoconv produces docs - should fix this)

Poppler (pdftocairo, pdfinfo) is used.  http://poppler.freedesktop.org/

A working but uglified pdf.js integration exists, which is enabled by
setting pdf.pdf_js=true mediagoblin_local.ini (disabled in mediagoblin.ini)

Adds one test to the test_submission test suite, and another separate test_pdf suite.
The tests are only run if media_types.pdf.processing.check_prerequisites passes, so
the test suite will not require any extra package.

TODO: make test suite say 'skipped' in that case instead of just 'ok'

Signed-off-by: Alon Levy <alon@pobox.com>