Refactor to require assignment of code to asana ID

Our hubot communicates via HipChat, which uses different short codes to
refer to people other than their full names. For example, to assign a
task to me, I would do:

    todo: @trussell Do something soon please!

I've refactored to require first telling hubot which asana ID maps to
which 'person', rather than attempting to do fuzzy-finding (Which
didn't work for our case).

You assign like:

    @hubot todo alias 1251251251512 @trussell

And you get the asana ID of the users by doing

    @hubot todo users

I've also adjusted the constants default values to be more meaningful,
and fixed error reporting for the latest version of the Asana API.