Always return unicode strings from pdftoxml.

The pdftoxml method on the old scraperwiki site returned
unicode strings.  Change this version to do the same by
interpreting the byte stream from pdftohtml as UTF-8.

Fixes issue #38
Merge pull request #78 from petterreinholdtsen/pdftoxml-utf-8-second-try

Always return unicode strings from pdftoxml.