Add scheduler

This pull request adds a scheduler for the server side of centinel. Basically, this adds a command line interface to specify the countries you want an experiment to run on and how often you want to run the experiment. The provided code will enable you to copy the correct experiments and data into the client folders while only dealing with a per country interface and enables easy management of the scheduler that will run on the client side.

@rpanah, would you please review this?
Why 30 days? In reality, a client that has not been heard from in more than 1 day should be excluded. Optimally, this should be made configurable in the future.