Support for Security Token Service

## Work in progress: Please, do not merge

This is a PR to add support for the Security Token Service, aka STS.

We tried to keep the coding style as close to yours as possible, but we're welcome to make corrections. 

For the `policy` classes, we have been strongly inspired by [seratch/AWScala](/seratch/AWScala), but we have avoided extending AWS classes. We are welcome for other approaches to this as well.

There are other methods still to be implemented. This is just to start a discussion.