aws-env doesn't clean up old credential files

[ketzacoatl]

During normal operation, aws-env is writing temp files with credentials. The creds go stale, so that's not really a problem. The problem is that aws-env will keep adding more and more of these files, and after a while you have thousands of them in your ~/.aws-env.

We should probably make note of this in the README, whether we do more than that is a good question.