Enable BasicSessionCredentials, given token

[elliot42]

AWS differentiates BasicAWSCredentials, vs. BasicSessionCredentials.
The later requires a session token, while the former does not.
Session credentials are required for features such as IAM, which
allows EC2 instances to retrieve temporary creds and access S3 without
having to hardcode a real user's creds.

This commit adds support for BasicSessionCredentials. Any call
that previously would have accepted a cred map:

{:access-key "...", :secret-key "..."}

will now also accept:

{:access-key "...", :secret-key "...", :token "..."}

and will authorize with BasicSessionCredentials if given a token.

[elliot42]

There have been a few attempts at this in the past--I'm not quite sure what happened to them / why they were not acceptable. I've tried to make this the smallest, simplest possible change. Please do let me know feedback if there's some way I can improve this so it's acceptable. Thank you!

[weavejester]

I think previous attempts either exposed the underlying Java classes, or possibly got overlooked.

This particular commit looks fine...

[sw1nn]

This seems like the simplest solution to the various PRs, #21 #28 #40, I'm going to merge this and close the other requests.