Added Freemium tier

We had initially decided against freemium as many new SaaS companies say its a timesink, but looking over our offering, it makes sense that Time Tracking should be free.

If you want to move over to invoicing through ZimTik, you'll need the Basic package. If you need multi-currency invoicing and many customers, you'll need the Professional package. In both cases, you should be earning a lot more than we charge you, so win/win.