Port argument to constructor was being ignored.

[ianjosephwilson]

• Fixed port argument by setting it to self.port.
• Switch findPort to return the port so logic in if statement in constructor is more explicit.
• Added assertion because an empty port raises a misleading exception when subsequent trying to open an empty port.

[thearn]

Made some changes to arduino.py since you made your modifications, which cause a conflict. Please check out the latest changes to master