Read .aws/credentials file

[leeper]

Add functionality to sign based upon .aws/credentials files of the form:

[default]
aws_access_key_id = ACCESS_KEY
aws_secret_access_key = SECRET_KEY
aws_session_token = TOKEN

[Alice]
aws_access_key_id = Alice_access_key_ID
aws_secret_access_key = Alice_secret_access_key

[Bob]
aws_access_key_id = Bob_access_key_ID
aws_secret_access_key = Bob_secret_access_key