refactor to move features like i2c, serial, etc to separate modules

[soundanalogous]

firmata.js is starting to get too big to easily manage. I think we should break out some of the features into separate modules. Initial candidates are things like I2C, Serial and the SPI support I'm working on.

[soundanalogous]

I'm not sure yet how this would impact upstream applications like johnny-five or other projects that depend on firmata.js. Maybe the firmata.js module could simply imports the required modules kinda like johnny-five.js does. Those modules could be:

firmata-core.js (digital and analog I/O, transport, etc)
firmata-servo.js
firmata-stepper.js
firmata-i2c.js
firmata-serial.js
firmata-spi.js
firmata-onewire.js
etc