Warn & continue if Arduino serial connection open fails

[tjanson]

Many users have run into the "cannot open /dev/" error message that so far occurs when the duino serial connection cannot be established. That message isn't quite self-explanatory, and it's not obvious why an Arduino should be strictly required in the first place.

Now:

• a meaningful warning is displayed
• Heimcontrol does not quit immediately (though it's possible that it'll crash down the road if something expects the Arduino Board to be present; this could be improved further)

In other words, you can finally run Heimcontrol without an Arduino.

Depends on ni-c/duino#4. Actually doesn't, uses my fork instead. This can be changed if the PR is accepted. Fixes #64, #50, #12 & many similar.

[ni-c]

I have changed the duino source in the package.json back to my fork: https://github.com/ni-c/heimcontrol.js/commit/c261d0aaa85ec27f612fbbe41167856dc8cc05a2