Fix "File Not Found" after export.

When `org-ehtml-cached' exports the file make sure the full path is
returned.  This prevents 404 errors when the html file is generated.

* src/ox-ehtml.el (org-ehtml-cached): Expand file name.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #5 from ruediger/fix-gen-file-not-found

Fix "File Not Found" after export.