NllTransformer -> AsNumpyArrayTransformer.
This change was done in order to allow the framework to work with dataframes as input as well as with numpy arrays.
Extracted the transformation part into a new class (Transformer).
Added ignore keyword.
Added cleaner.
Added basic nan treatment (mean and drop_rows only).
Allowed the target argument to be categorical (assigns a different number to every different class, starting from zero).

[Bugfix] Dataframe index is now reset whenever a row is dropped.
Minor refactoring + Import cleaning.
Input data source to abris can now be either a file like object or a pandas.DataFrame.
Added more methods to nan treatment: median and mode.
Nan treatment now supports categorical missing values in numeric modes using the mode.
Continuous keyword on the configuration file changed to Numeric.

Added a description of the data model to the readme.
Moved Features to a new file.
Cleaner now trims text features.