RFC: Only columns of strings should be made factors.

As I understand the spec and the outline of the the readtable method code,
only columns of data files that have all strings should be made into factors
given that the makefactors boolean in the method is active.

This logic is implemented here.
Merge pull request #317 from mweiden/factors-by-column

RFC: Only columns of strings should be made factors.
Switch require(test.jl) to using Base.Test
Fix test suite
Remove the model_frame and model_matrix methods. Apply vector in model_response.
Add pool and pool!
Generalize pool and pool!
Merge pull request #320 from HarlanH/df/form

Remove the model_frame and model_matrix methods.
Minor tweaks in gl and for model_response
Update docs
Fix missing underscores
Remove apostrophes that upset Jekyll
Fix markdown problems
Fix markdown problems
Revised sections 1 and 2 of the docs
Switch merge to join and use keyword arguments

Revise documentation for recent changes
Move documentation to the README
Update docs
Fix merge
Introduce filter as an alias for subset