Customizing categorical variables

[wleoncio]

Summary

This PR does some more work on issue #2 by allowing the user to define which variables are binary and categorical.

Notice the discretionary rule for auto-defining a variable as multi-class (it muse have between 3 and 5 unique values). Hope this makes sense.

Commits

• Recode all Pos/Neg columns to -1/1 (Define general transform function #2)
• Added bin_vars argument (Define general transform function #2)
• Added cat_vars (Define general transform function #2)
• Updated version number
• Simplified test code
• Added auto-processing of categorical vars (Define general transform function #2)

Left to do in #2

Allowing the user to classify a multi-class variable as a binary one (see #2 for details).