New styling and colour scheme for 10.12.

[tatey]

The widget was originally built on 10.10. The whole notification center got a significant revamp in 10.12. The widget still works, but it doesn't look as good as it could.

I'd be happy to drop support for < 10.12 to make it easier to maintain going forward. My suggestions:

• Use light colour scheme
• Fix cell alignment
• Update screenshots in README.

[segan5]

Agree with all your suggestions.

Submitted a PR for the light colour scheme, its just a field you need to add in the info.plist as per https://stackoverflow.com/questions/39811118/sierra-notification-center-widgets-background-colour

I also expanded white values you can set in the widget to be in line with the iOS app. Feel free to reject this if you don't think its appropriate.

[tatey]

Thanks again for your contribution in #23. As agreed I'm splitting out the alignment into a new issue.