Explicit callback handling with optional http method, and clearer callback data param names.

* Allowing override of default callback method by passing callback_method on form helper.
* Renamed "post" and "as" options to "callback_url" and "callback_param" for clarity
* Updated doc with instructions for updating an existing model

Related to https://github.com/waynehoover/s3_direct_upload/issues/44

Note: The renaming of the "post" and "as" options is a breaking change, but one that I feel is worthwhile for added options clarity.