constructor for ModelFrames based on Terms and DataFrame
predict method for dataframe regression models and dataframe (vs. matrix) predictors
shallow copy of Terms and a function to remove response from Terms
during predict, create new ModelFrame without response
post-review cleanup

* style
* remove extraneous Terms copy constructor and in-place remove_response! method
* remove extraneous ModelFrame(::ModelFrame, ::AbstractDataFrame) constructor
Merge pull request #679 from kleinschmidt/dfk/predict

Add predict method using DataFrame predictors