Handle prop updates in FineUploader shim

[sohkai]

ReactS3FineUploader ignores any prop updates that are related to FineUploader after it's been initialized. This makes it quite difficult to dynamically change the upload url afterwards, for example.

This is somewhat hard to handle because some parameter changes, like the upload url, would cause any current uploads to go into an inconsistent state, so we'd probably have to nuke the uploader and start from scratch again. As well, FineUploader isn't known for having the fullest api, in terms of changing its configuration after initialization.