Display a summary if there are more than 15 columns. Fixes #373.
Decide about dataframe printing more intelligently

Decide whether or not to print a dataframe based on whether its printed
width is more than Base.tty_cols().
Use mapreduce instead of list comprehensions to calculate max column length
Merge pull request #376 from jvns/master

Display a summary if there are more than 15 columns. Fixes #373.