Rev 3 Changes

[adkinsjd]

• Battery backed RTC on the shield
• Route VIN directly to an external interrupt pin (with some kind of voltage step/protection)
• Route Sensor power enable pin
• Make all peripherals off by default
• DO NOT connect the apn switch to the WKP pin. Maybe connect usb voltage to wakeup pin.

[nklugman]

• potentially TPL5000 or some other external wakeup timer to WKP pin
• make it more difficult to plug in particles offset on the headers
• add temp sensor
• low voltage interrupt? something to tell the particle to stay asleep that is more of a hardware trigger?
• new sd card sockets
• ye-sheng 60hz detector (likely for rev 5 or something)
• lower cost imu?