Summary new (final, hopefully)

Re-written Summary() class in the summary2 module. Many regression models are given summary2 methods that use the new infrastructure. By default, the summary() method of each model uses the old summary functions, so no breakage is anticipated. Also includes summary2.summary_col() method for parallel display of multiple models.

In time, I hope to: 

* Improve the look of summary2() output
* Remove the SimpleTable dependency by writing a much simpler, more flexible and robust ascii table function.
* Deprecate the old summaries to use the new infrastructure