* added new flags `plan-backend`, `plan-s3-bucket`, `plan-s3-prefix` and deleted `s3-bucket`
* interface `plan.Backend` that is implemented by `file` and `s3`
* simplified s3 code
* didn't end up following my suggestions in #30 since storing stuff in metadata requires you to `Get` the object *first* and then use the metadata. By parsing the `Key` to get repo, pull, path, and env, it skips an initial `Get` step, and I can download directly to the correct directory
* allow users to specify `application/json` or `application/x-www-form-urlencoded` for the webhook delivery type
* remove sending of special header for pull request api (fixes#11)
Closes#30 and #17 and #11
