Aws-sdk-signers

Feature
* Added new ``content_checksum_enabled`` parameter to SigV4SigningProperties.

This will enable users to control the inclusion of the `X-Amz-Content-SHA256`
header required by S3. This is _disabled_ by default, so you will need to
set this to `True` for any S3 requests.

Bugfixes
* Fixed incorrect exclusion of `X-Amz-Content-SHA256` header from some requests.

Features
* Added SigV4Signer to sign arbitrary requests sychronously.
* Added AsyncSigV4Signer to sign arbitrary requests asychronously.
* Added example SigV4Auth for integrating directly with Requests'
``auth`` parameter.
* Added SigV4Signer for integrating with the AIOHTTP request
workflow.
* Added SigV4Curl for generating signed curl commands.