Added Fusion PGNs, added missing info to some enviro PGNs
Merge remote-tracking branch 'upstream/master'
Reverting my huge pgn.h commit
Added lookup table for Temperature 130312

Set temperature 'known' flag to true
Corrected 130313 and 130315 based on corrigenda published by NMEA
Added outline of Fusion proprietary PGNs
Merge pull request #14 from timmathews/master

Tim Mathews sent in some improvements:

Fusion PGNs
Improvement to PGN 130312 "Temperature" (lookup table added)
Improvement to PGN 130313 "Humidity" (NMEA corrigenda based)
Improvement to PGN 130315 "Set Pressure" (NMEA corrigenda based)