[2.0.0] trigger assigned to escalation policy

This addresses my concerns in https://github.com/hubot-scripts/hubot-pager-me/pull/12#issuecomment-61838790

When assigning incidents, the order goes like:

* exact chat username
* first escalation policy if exactly one match
* escalation policy with exact name if more than one match
* first schedule if exactly one match
* schedule with exact name if more than one match
* otherwise, prompt user to specify something else