Enable BasicSessionCredentials, given token

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.
Merge pull request #49 from elliot42/session-credentials

Enable BasicSessionCredentials, given token