Feature: Bedtime calculator

Adds a bedtime calculator to each alarm. The calculator works by
counting backwards in sleep cyles (90 minutes), then calculating the
times you'd need to go to sleep in order to wake up in between them. It
can help you wake up feeling less groggy. The calculator displays four
suggested sleep times ranging from 4.5-9 hours of sleep.

Change-Id: I7ad932da7955d8f058392562ffc85c9af0f5206f