Escape special characters in file paths when attaching files

Re http://forums.zotero.org/discussion/5311/2/importing-endnote-libaray-including-pdf-attachments/

The `#` in the path is interpreted as the fragment separator (I think) but it is allowed in file names and directories. I think there might also be issues with `?` and perhaps `+` and `&`.

There might be other places in Zotero where this has to be done.