Restore compatibility with Prettytable < 0.7.2

Starting with 0.7.2, PrettyTable changed default on how empty
tables are rendered. Before they were completely ommitted, now
the default is to show the table headers and decoration, just
no content. Restore the previous behavior to stay compatible.
cmd2 0.6.5.1 was released, and is compatible
Merge pull request #4 from dirkmueller/prettytable

Restore compatibility with Prettytable < 0.7.2
Merge pull request #5 from dirkmueller/more-strict

cmd2 0.6.5.1 was released, and is compatible