Merge pull request #331 from tanmaykm/master

tracking fix: JuliaLang/julia@f38c2b8d0eff54efabc8c6a21f920aa71ddf399d
Much faster writetable implementation
Revise I/O code
Update IO
Handle the empty string when quoted
Add the ability to modify a DF in place based on an I/O buffer
Fix deprecation of mege
Check for columns that are all NA's
Convert DataArray element type
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