Fixes #36 Duplicate files and deletion issue

Hopefully this pull resolves the overwriting of files on upload.  It does a best effort check early in the upload process to avoid filename collisions.  There's a small window to clobber files if multiple people are uploading the same filename.  The standard wordpress avoids this using the wp_unique_filename call in wp_handle_upload.  Unfortunately we have no ability to inject a stat-test into that so the prefilter seems the least obtrusive.