Update documentation/code for pullups and I2C Bus Speed

[qdot]

Original comment: http://disq.us/6iwldk?imp=cce7d05a-a21a-43d8-941c-a3e31ea21eee&thread=465932670&zone=email_notification&event=shortener_click

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I just got my Arduino Mega controlling the MyKeeopn, and a few notes that people might find useful:

• didn't need a 5v 3.3v converter, because I2C uses open-drain pins. Just connected both the I2C lines to the +3.3V supply from the Keepon through some pullup resistors
• needed to manually change the Arduino I2C frequency from 100khz to 25khz (which is the Keepon's I2C freq) by adding this line before beginning any Wire commands (adds a 4x prescaler to the TWBR register):
  sbi(TWSR, TWPS0);

Thanks qdot for all the info, it was really helpful!

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