Changed key sysex methods to be public

I tried to get fancy and add a PR to an existing issue, but I didn't get it quite right. This is a preliminary PR for #3. I realise it will change soon, but it will really help me. 

If you can push this into master and release it, I can then release the dashboard without having to use a forked clj-firmata dependency. No rush, but when you get a chance. Thanks :)