Added ms5611 barometer code.
MS5611 add more error checking.

Added some delay in ms5611_init(), which caused failing of initialization.
MS5611 add missing header guards.
MS5611 fix doc.
Add baro command.
MS5611 moved code into src/sensors/
MS5611 simplified internal structure, removed waiting for ADC.

Chages from last code review.
MS5611 update test code.
MS5611 add missing static.
MS5611 improve comments and variable names.
MS5611 correct prom read length.
Merge pull request #1 from nuft/barometer

MS5611 barometer code.