Add calibrate() method to estimate sensor bias

[brusshamilton]

• Adds a calibrate() method to estimate sensor bias. Based on the "USING SET AND RESET TO REMOVE BRIDGE OFFSET" section of the datasheet. These estimates are applied to correct measurements.
• Adds additional member variables to hold the bias estimate. Until "calibrate()" is called these parameters default to the previous hard coded value (1 << 19) so that the previous behavior of the class is retained for existing users who do not take advantage of this feature.
• Pulls out magnetic data reading into a separate internal function so that it can be reused by calibrate.