Add missing endpoints

[philnash]

Looks like Bitly have added some endpoints since this was last released. Here are the new ones I think are missing:

Bitlinks

• [ ] Metrics for Bitlink by Cities
• [ ] Metrics for Bitlink by Devices

Groups

• [ ] Clicks by Group
• [ ] Click Metrics for a Group by Cities
• [ ] Click Metrics for a Group by Devices

Organizations

• [ ] Plan limits for an Organization

[philnash]

Turns out these are pretty much all premium endpoints. Added them to the README and that will do for now.