Add support for syncing with FreshBooks

[imiric]

This would possibly involve an interactive environment, similar to git add -i (see Python's cmd module for this). Here you would be able to edit all entries, assign to FB tasks, split times, etc. And when you're done, the sync would be processed. If successful, the changes would be saved to the sheets DB, and marked as "synced". Otherwise, you'd be thrown back and have the chance to fix any issues.

Extra points for making it generic to be able to sync to any service, but this is not a priority.