Truncate messages that are too long to send

Twilio has a 1600 chatacter limit on the message to send.

Instead of having the API call just fail when trying to send the
message, send as much of the message as possible with a short
message on the end indicating that it was truncated.
Add robot name to the start of the message if it's not there

This is so messages don't have to start with the bot's name for
the bot the respond to them.