init preprocessing/ module with an ItemsetEncoder

[scikit-mine]

init preprocessing/ module with an ItemsetEncoder

[remiadon]

two things here

1. pandas already provide some utilities to convert labeled indexes to their position, so better not reinvent the wheel
2. defining this "convert to integer" step as a preprocessing step makes an external post-processing step mandatory in most cases (convert integers back to labels)

From my own experience it's not very costly to do this as an internal step, eg in SLIM

So closing this