Ignore hubot user

The Kandan Hubot adapter seems to have been hardcoded to ignore all messages from Kandan user with id=4. Not sure why this was. I added an environment config variable to ignore based on username instead, since it'll still be unique, and since it makes it easier to ignore Hubot with its default username.

Note that this pull request is on top of #7, only because it adds to the section of the Readme that was created in that pull request for optional config variables.