Edit support for messages

[kristenmills]

If I edit my last message to something hubot understands (and content changed):

• if hubot didn't respond before, hubot responds now
• if hubot did respond, hubot updates his response

[kristenmills]

Another possible If I edit a message that hubot understands (and content changed):

• if it was the latest message and hubot never responded, hubot can respond
• if hubot responded in the past, It will edit his response.

This way hubots response doesn't ever look out of place.