Changed the GLM API for consistency

I have made some changes in the API so that we can integrate seamlessly with Statsample::MLE.

The constructor now accepts an entire dataset which includes the 'y' value  of the regression, so that loading data from files will be fecilitated. The user then specifies the column name that should be used as the dependent variable. This I think adds more flexibility. 

There was a bug in the original statsample-glm that if intercept values wernt supplied as the first column of the dataset, the  regression would fail. This was handled in MLE by later appending  a column of intercepts, which is what is done here too. We now give the user an option in the constructor to determine the value of intercepts, so that such type of bugs will be altogether avoided.

Please evaluate the code and let me know.

Cheers!
I want to use `Kernel.const_get` for calling dynamic classes. `eval` isnt safe because it can execute any ruby code you pass to it.

Versions prior to 2.0.0 dont support `const_get` unless you chain them for each constant you want to call. Plus 1.9.3 is at the end of its life anyway.