Allow upload to S3 bucket root

This now works such that members of `qwwx` are uploaded to the root of the bucket:
``` bash
$ file qwwx
# => qwwx: directory
$ travis-artifacts upload --path qwwx --target-path ''
# => I, [2013-05-20T08:12:15.346222 #2779]  INFO -- : Uploading file /Users/d.buch/workspace/travis-artifacts/qwwx/bar.txt to bar.txt, public: true
# => I, [2013-05-20T08:12:16.114617 #2779]  INFO -- : Uploading file /Users/d.buch/workspace/travis-artifacts/qwwx/foo.txt to foo.txt, public: true
```

Closes #2