More test code; minor fixes to is.named() and documentation

1) New test code for assert-scalar.R and assertions.R
2) Changed is.named() so it uses `na.rm=TRUE`. See issue #19 
3) Documentation typo fixed
I don't think the logic is quite right because `all(logical())` is TRUE. It would be better to do `all(!is.na(nm) & nm != "")`