simplify device wakeup, per the datasheet

datasheet calls for simpler wakeup routine
changed/clarified comment regarding wakeup sequence
added readSensor method

the readSensor() method allows easy reading of
both the temperature and humidity in a single call
fix handling of negative temps

highest bit in temperature registers is sign
1 = negative, 0 = positive
added delay betweent request/response

datasheet calls for short 200ms pause between
request to read registers, and actually reading the data
Merge pull request #1 from mschoch/general-cleanup

General cleanup