I built this app for my own use in the gym, for rest intervals.
There's three simple options and the app includes a configuration screen available from Pebble phone app allowing you to control the rest interval pre-sets.
Currently only available to be installed in development mode - coming to the Pebble "store" soon.
If you want to build it yourself, I highly recommend using cloudpebble.net.
Yes, you can also use this for Pomodoro!