Updates to the redesigned RangeFinder library

I made most of the updates from the development call:
- Deleted the Px4 rangefinder example since it was out of date
- Removed the Auto type to address the same device being detected twice
- Changed the RangeFinder's parameter prefix from SONAR to RNGFND
- Do a 2 byte transfer in the PulsedLight distance read
- Added a destructor in the PX4 device to close the file descriptor
- Renamed the sonar test in ArduCopter, added more details for each device

I'm still working on the hot swapping ability by keeping track of Last Type but wanted to submitted these in the meantime. If any of the above changes need modifications, just let me know.