[2.0.0] trigger assigned to escalation policy

[technicalpickles]

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