Modify Describe function

1) Print out NA count/percentage for all cases. It feels more
comfortable to have NA always printed out explicitly, rather than
having to assume that NA = 0. Also, calculated NA% to give context
about data completeness

2) Added unique count for character string scenario, to help user
determine whether column should be pdv or if column is a key (i.e. high
cardinality column for joins, as opposed to factor)

3) Fixed formatting for character case to remove colons to match
numeric table format
Merge pull request #337 from randyzwitch/master

Modify Describe function