Fix file exsitence check key to use relative path from dir of S3.

s3sync should use relative path from dir both of local and S3 to check file existence, but not now.

On second time I try to sync my local dir to s3, s3sync uploads all files in local and deletes all files which existed in s3 before upload.  

This pull request may fix same bug reported by [spiffxp's pull request](https://github.com/clarete/s3sync/pull/31).
I wonder how such critical bug remains.