Added X-Amz-Date creation

Added a new export, headerXAmzDate which takes a JS Date object (or uses Date.now() by default) and converts it into the X-Amz-Date header format.

Mostly useful for signing requests using fetch() since you can't specify a Date header in that case due to it being a Forbidden Header Name.

Fixed a module path

v2.0.0 came out with an incorrect module path in package.json, this has been fixed.

Signed query support

You can now sign requests via the query string! See README.md for details.