Shimmer heart rate driver update

Greatly simplified. Removed unused, inaccurate, functionality. Now the timestamp of when the heart beat is detected is passed to the higher level application rather than trying to determine the beat rate. Remove spurious beats added by some (non-rmcm01) coded sensors.
Shimmer MSP430 I2C bug fix

Change name of I2CDR register to I2CDRB (as former is deprecated in msp430-gcc v4.6.3).
Shimmer mag driver update

- Increase startup delay in Init.init() function from 5ms to 10ms (datasheets says chip takes 8.3ms to startup)
- Fix errors in setGain() command