Support for Session / Federation token credentials

[jantman]

One user ( #122 ) was trying to authenticate with a Session or a Federation token, i.e. with the data returned in a STS Credentials response.

If some organizations are using things like STS to give limited-duration leased credentials to their users, then it is probably a good idea for awslimitchecker to support these, so people can evaluate it without needing administrative approval.

[jantman]

Using 0.7.0, STS credentials work fine with awslimitchecker if I export the AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_SESSION_TOKEN environment variables.

This just needs to be documented.

[jantman]

This was just released in 0.8.0, which is now live on PyPI. Apologies for the long delay before this release.