add signoff message at end of office hours

[jopecko]

When using office hours, the bot posts the final challenge to the channel even though it may fall outside the office hours window. This patch first checks to see if the next challenge will occur within the posted office hours and if not instead prints a final message signaling the end to the day's workout activities.

[slinlee]

This would be great for #27 if you had it show the summary of the workouts at the end of the day.