Adding support for amazon STS

[shaunenslin]

Hi, I have added support for amazon STS for assuming an IAM role.
Very handy for software being developed and rolled out without need for usual credential's.

If you can give me access, I can submit the pull request.

thanks

[brainstorm]

@shaunenslin You don't need any access to submit a pull request. I'm also interested in STS since I'm testing this lib right now :)

[tejasmanohar]

Have you seen athena.Open({Session: aws.Session... })? https://godoc.org/github.com/segmentio/go-athena#Open

You should be able to make an AWS session out of STS credentials and supply that, right? I believe we do that somewhere...