Support for temporary credentials

[senior]

When trying to use the ocaml-aws library, I couldn't figure out a way to supply temporary credentials using the API as it is. What I needed to support temporary credentials, was to add a "x-amz-security-token" header with a supplied token. I think it might be as easy as adding an optional parameter to sign_request that will add the appropriate header, but I think that would then need to be surfaced in places that is being called, like Aws_async.Runtime.run_request.

More info on temporary credentials:
-Making Requests
-REST authentication

[tmcgilchrist]

I haven't looked into supporting the temporary credentials functionality yet. I'll put it into the backlog of feature request. My focus with this library right now is shaking the bugs out of the currently supported bindings, like #75

If you feel strongly about it please submit a PR. Cheers 👍

[jrochel]

#99