Add the notion of billable hours

Description

Companies often use contractors to do some work. They also work for their customers. For those two use cases, they need to mark some hours as billable, so they can charge for it.

Feedback from Hacker news (https://news.ycombinator.com/item?id=13020497):

We track times to bill clients in service requests

Proposal

Add the concept of billable hours.