Remove arduino.zip from version control

[nicolaspayette]

• arduino.zip should be downloadable from https://github.com/NetLogo/Arduino-Extension/releases.
• arduino.jar and jssc-2.6.0.jar should be fetched by sbt (that's Set up sbt build #2).
• The example model and the sketch file should probably just be removed. They're both in https://github.com/NetLogo/models/tree/master/IABM%20Textbook/chapter%208 now, and should not be kept in two places.
• README.md should be updated to reflect all those changes.

[nicolaspayette]

The sketch file in the zip seems to have more detail than the one in the library. This needs to be sorted out before removing. The example model should also be checked.