Modify describe function for additional metrics

1) I feel like it will be more useful to always print out the number of
NAs, even as zero, to be explicit. Also added NA%, so that user can
understand how complete a numeric column is

2) Added unique values for characters, to help user understand whether
the column is a candidate for a pdv or whether column represents an ID
type of column (high cardinality). Also removed colons and changed
spacing to match numeric summary (looks right on OSX)