Allow uppercase names in Asana task assignments

Because the code does a `toLowerCase()` on the names coming from API data (https://github.com/github/hubot-scripts/blob/master/src/scripts/asana.coffee#L79), Hubot command necessarily has to take a lowercase form or the equality (https://github.com/github/hubot-scripts/blob/master/src/scripts/asana.coffee#L80) doesn't return true 

`task: @Taylor blah`
> Unable to Assign User
> Task Created : blah

`task: @taylor blah`
> Task Created : blah : Assigned to @taylor

If we `toLowerCase()` input data as well, the check should pass and using lowercased names in task assignments should not longer be necessary.

Cross-pulled to https://github.com/idpro/hubot-asana/pull/2