Support arguments (canonicalQueryString) and UNSIGNED-PAYLOAD

[smiba]

Slightly messy PR, sorry about that

These changes provide

1. UNSIGNED-PAYLOADS when doing a PUT request where ngx.var.request_body is empty (might happen when it does not fit in nginx body buffer)
2. canonicalQueryString, allowing to use this library to sign requests containing query strings
3. Some README modifications, the old readme did use an incompatible request_uri (it would contain the query string which is not valid!)
4. Fixes for the warnings for race conditions on k_date/region/service
5. A way to manually specify region and service, for when this library is used for a non AWS provider

To do:

1. Handle QueryString's without value (?option instead of ?option=1 etc.)

[smiba]

0b08243 fixes the empty QueryStrings :)

Last thing to do is be more smart about routing requests like ?delete (bulk removals), but this is gonna require a fair bit more work, possibly outside of this library

[ssalinas]

Hey @smiba , I'm not wure when I'll have a chance to test this in our own infra before merging. I take it you are able to use your own fork for now and aren't blocked here?

Thanks for the contribution by the way!

[smiba]

Yes! We're using this and it's been working fine, I specifically made these contributions to the code for our infra

(Even made a little write-up which includes using this library: https://medium.com/@PurpleDug/woof-tech-routing-s3-endpoints-and-doing-a-live-migration-on-a-production-environment-as-a-treat-8fa2bbd6784e)