TMPDIR overlaps on OS X

When creating `TMPDIR` on OS X it doesn't seem to be unique on each run of the script, especially if using the "zip" type. The fix for me was to initialize `TMPDIR` as follows:

``` Bash
TMPDIR=`mktemp -d 2>/dev/null || mktemp -d -t '$PROGRAM.XXXXXX'`
```

This fix is safe for both Linux and OS X.
