Add support for webhooks

[lubos]

1. There is new API going to be added within a few weeks. It won’t support webhooks but they could be easily added on top of that.
2. How do you imagine multiple contacts to work? Have a different contact for quote sending, different contact for invoice sending?
3. I imagine in future, it will be possible to create a custom tab but it would just contain a list of custom objects. E.g. you want to track phone calls, so you could create a custom object “Phone Call” with custom attributes and it would show as custom tab.