Support text:span in functions

Created originally on Bitbucket by asplinsolutions (John Asplin)

Was already merged in Bitbucket before import, marked as merged by the import user

I've been working with templates derived from MS Word documents and have found a lot of instances where text:span tags have been added inside of the text:p tags by Libreoffice.

When trying to process the template it triggers an exception with the error "We handle urls in tables or text paragraph only".

This PR treats text:span the same as text:p in this particular handle_link function and this overcomes the error I was experiencing.

I've tested against tox ("tox -e pep8,py27") and am currently using this modification in a custom docker image for my own project.

Happy to hear if this is the wrong approach :)