Fix creating empty 'directory' on S3 when not specifying a path

Currently if you don't specify a `path` to the S3 plugin, it defaults `path` to the empty string. Then, when `File.join(path, File.basename(file))` is run, a file named `foo.ipa` becomes `/foo.ipa`. This causes the uploaded file to have a double slash in its URL:

https://s3.amazonaws.com/hz-sdk/bar//HeyzapSDKTest.ipa

In S3, this creates a directory without a name where the double slash is. You can still click on the empty space and traverse to the file, but it seemed to be clearly a bug.

![screenshot 2014-10-08 18 06 15](https://cloud.githubusercontent.com/assets/1274145/4569813/597cfbc0-4f51-11e4-95af-9c90a5bb0ca8.png)

Fixing this might be a breaking change for anyone relying on the old behavior, though :(