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.
require requirejs (fixes #52)

Note that I have no idea what I'm doing; I know nothing about requirejs.
I saw this post:
https://stackoverflow.com/questions/14548369/requirejs-relative-paths#comment20294321_14548369
and I saw that `requirejs` is undefined. So let's define it.

Fixes #52.
Don't crash on req w/o cookies (redir to login)

When handling a request without cookies, i.e., with
`typeof req.headers.cookie === 'undefined', detect the problem and redirect to
login rather than crashing.
Fixes #76.
Merge pull request #97 from tjanson/arduino-connect-optional

Warn & continue if Arduino serial connection open fails
Merge pull request #99 from tjanson/fix-52

require requirejs (fixes #52)
Merge pull request #100 from tjanson/fix-76

Don't crash on request without cookies (redirect to login instead) (fixes #76)
Changed package.json back to original duino repository
Merge remote-tracking branch 'upstream/master'