AWS credentials from IAM configuration

When the gem is used from a Rails application stored in EC2, there is a feature that allows to use temporary access keys that are regenerated every hour. The aws-sdk takes care of this by itself, but s3_direct_upload gem doesn't work if no S3 are provided. I added some code so the config object checks if the keys are there, and if not, it tries to grab them from AWS object (if exists).