Led class overhaul (see commit detail)

- `on` always turns an Led "on", whether digital or analog
- `off` always turns an Led "off", whether digital or analog
- `pulse`, `fade`, `strobe` can be _stopped_ (ie. the interval is terminated) with `stop`
- `pulse`, `fade`, `strobe` can be _turned_ _off_ (ie. the interval continues, but the Led appears "off") with `off`
- `pulse`, `fade`, `strobe` can be _turned_ _on_ (ie. the interval continues, the Led is either "on" or "off", based on the operation) with `on`
- multiple calls to the same method do not pile up
- unnecessary dependencies on instance methods are removed in favor of direct calls.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
v0.6.14

Led overhaul

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>