Made UserName optional

Following
http://docs.aws.amazon.com/IAM/latest/APIReference/API_GetUser.html
The UserName argument in iam.GetUser() is optional.

In the case where the UserName is omitted, the response returns information about the user making the request. This is incredibly helpful when running code on an instance with an IAM role.

Fixes #192
Merge pull request #193 from drombosky/td-issue-192

Made UserName optional