Added a new feature: post-processors.

* Added `default` post-processor (JSON string).
* Added `json` post-processor (JSON Object).
* Added `base64` post-processor (base64-encoded JSON string).
* Added `arraybuffer` post-processor (binary ArrayBuffer).
* Fixes #2.
Merge pull request #3 from jussi-kalliokoski/feature-post-processors

Added a new feature: post-processors.