Add `iam_user` module that supports check mode
Based entirely on `iam_group` with check mode support added.
Does not support management of groups, so as not to interfere with the
`iam_group` module.
* Update the copyright
* Remove e.message from exception handling since it is not available on python 3
Dag Wieers
Abhijeet Kasurde
John R Barker
Ryan Brown
Josh Souza