Refine 7-segment backpack to use inverted digits, and filled out clock example

[tomgidden]

This could benefit from better handling of colons and dots, but that's already affected by which backpack / display is used.

[tomgidden]

No problem; thanks for the credit! To derive the digits I just figured out which segments to light for each digit and converted binary to hex with a calculator app. More trial and error than anything else though :)

Note, I've added a similar change to another Adafruit repo for these displays that seems to be completely different code: adafruit/Adafruit_Python_LED_Backpack#5 As I'm now also using an alphanumeric display backpack on my project I might also take a swing at doing invert for that display too, or maybe make a more generic change to the base class.

[ladyada]

This library has been deprecated in favor of our python3 Blinka library. We have replaced all of the libraries that use this repo with CircuitPython libraries that are Python3 compatible, and support a wide variety of single board/linux computers!

Visit https://circuitpython.org/blinka for more information

CircuitPython has support for almost 200 different drivers, and a as well as FT232H support for Mac/Win/Linux!