Don't explicitly call the setup() method of S3_Uploads

[fklein-lu]

The setup() method of S3_Uploads class is called directly multiple times in the code.

The first occurrence of such a call is in s3_uploads_init(), hooked to plugins_loaded. This means that by the time that the other calls happen, the method has already been called once at least.

Since the setup() method hooks up different other methods, it should only be called once. Since S3_Uploads is a singleton, we should move the call to the method into the constructor and mark it as protected.

@joehoyle since you know the plugin best, I'd appreciate your input.