Add Request Body Canonicalization specification

[tw4l]

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Fixes #141

This PR adds a new specification for Request Body Canonicalization (sometimes also referred to as POST canonicalization), which describes new consistent behavior between pywb and warcio.js, with fixes to bring them in line to the standard in https://github.com/webrecorder/pywb/pull/893 and https://github.com/webrecorder/warcio.js/pull/68.

The spec is based heavily on the draft specification written by @ato and published in the WARC Specifications repository at http://iipc.github.io/warc-specifications/guidelines/cdx-non-get-requests/.

At this point the description of how AMF bodies are transformed simply points to the pywb codebase and points out that the feature is only in pwyb and is considered experimental.

Perhaps optimistically, I set the publication date to next Monday :)