Greenhouse Board

This is an accessory board for a Raspberry Pi, designed to add a real time clock, sensors, 
and relay control.  The clock allows event scheduling even if there's no network connection,
though we anticipate having our greenhouse on the net full time.

The sensors include barometric pressure, temperature, and humidity using an MS-8607, and
a light sensor using a photocell and associated ADC.

There are 4 GPIO-controlled AC relays with LED state indicators.

The basic idea is to log environment conditions, present them over the web, and use the
relays to automatically control heating and cooling systems to regulate the environmental
conditions.