Startup improvements

There were some problems when trying to start the bot in two cases:

1) The bot was not connected at all
2) Johnny five attempted to connect to a Bluetooth device instead of USB

This pull request adds some basic checks for USB connections (tested with only one at a time, on a Mac with two USB ports) and exits in case it can't find anything connected to any of the available USB ports.

Please do a `npm install` after merging to install the serialport package.