Add activelow support for active low devices

I am started using active low relays with heimcontrol and in order to get them to switch on when pressing the on switch, i created an almost exact replica of the led method, but i have switched the on method to set the pin LOW and the off method to set the pin high.

You will also need to add activelow.js and update the index.js duino file in order for this to work.

├── node_modules
|   ├── index.js (must update file to include activelow.js)
├── lib
|   ├── activelow.js (Must add file)

These files can be downloaded [here](https://gist.github.com/schneiderr/71ea2fc8abebac03ebfa).