Add ui::LedPin and ui::LedDriver classes

[tyler-gilbert]

These classes will implement the Led class to control indicator LEDs.

[tyler-gilbert]

If this goes in UI, there should be a base UI object called Led. These should be named:

PinLed
DeviceLed

See #21

An argument could be made for creating hal::Led and hal::LedDevice as well.

[tyler-gilbert]

Maybe the UI object should be called "Indicator"