This makes it possible to customise the endpoint / hostname per request, rather than having the static / global state.

NOTES:
1. A great level of effort has been put in to make sure backward compatibility changes to be minimal, but there are cases that couldn't have been avoided. Sorry peeps!

2. Also some custom business logic for bucket region deduction has been removed, and has been replaced with a callback.

Other changes
* Reduce global / static state in S3 class
* Various efficiency tweaks
* Various formatting tweaks

* Removed the region deducer callback; RTFM for me!
* Added workaround to avoid 307 redirects by always talking to the correct regional-service-endpoint. This also improves performance overall as it requires less network magic in AWS.
* Fixed examples such that no code need to be modified in order to run them; takes parameters from environment.
* Tweaks to composer.json to fix autoloading and to specify extension requirements.
* Added export-ignore git attributes to remove dev artifacts from composer install when included into other projects.

Turned ON via an env variable SIGNATURE_DEBUG=ON