warn & continue if Arduino serial conn open fails

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.
Merge pull request #97 from tjanson/arduino-connect-optional

Warn & continue if Arduino serial connection open fails