Fix issue with relative paths to image files (images not incorporated into document)

For your consideration...  (I'm sure another, better slugify option must exist - for one thing, this reveals some info about local paths in resulting zip/docx)

The issues I saw were that:
 - images specified with relative file names would not get into document
 - it looked to me like two images with same names but with different paths *might* collide  (*I did not test this specifically)

To reproduce the bug, you can use commit 51aa224 of http://github.com/ransage/csv2docx/
(that commit has a crude, hard-coded test case, which is set up to work with virtualenv)