added sum_na() so sum with na.rm=T, but if all NAs return NA and not 0. cleared all warnings
added sum_na() so sum with na.rm=T, but if all NAs return NA and not 0. cleared all warnings.

Merge branch 'master' of github.com:bbest/ohiprep

Conflicts:
	Global/FAO-Commodities_v2011/data/FAO-Commodities_v2011_tonnes.csv
	Global/FAO-Commodities_v2011/data_prep.R
Merge pull request #44 from bbest/master

added sum_na()