Implement na_if

[hadley]

Equivalent of SQL NULL_IF, http://www.resolvinghere.com/sof/6220956.shtml

na_if <- function(x, y) {
  if (length(y) != length(x) && length(y) != 1) {
   stop("`y` must be length 1 or same length as x")
  }

  x[x == y] <- NA
  x
}

[hadley]

Also need to add SQL translation.