Implement support for LR4 rangefinder and perform minor cleanups to AP_RangeFinder

Major changes:
+ Implement LR4 rangefinder in AP_RangeFinder_LR4
+ Add example sketch for LR4 rangefinder

Minor cleanups:
+ Correct comment regarding barometer/sonar frequency in ArduCopter.pde
+ Remove duplicate comment in UARTDriver.cpp
+ Remove extra spaces and fix spelling error in AP_RangeFinder_MaxsonarI2CXL.cpp and AP_RangeFinder_SharpGP2Y.h
+ Remove unused function set_orientation() in RangeFinder.cpp