[BUG] Add amazon_digital_music to datasets init

[pablo-pnunez]

Description

Despite being implemented in cornac/datasets/amazon_digital_music.py the file is not listed in cornac/datasets/__init__.py.

In which platform does it happen?

All

How do we replicate the issue?

dataset = cornac.datasets.amazon_digital_music.load_feedback()

Expected behavior (i.e. solution)

cornac/datasets/__init__.py:

from . import amazon_digital_music

[tqtg]

@pablo-pnunez thanks for pointing this out. Great catch!

@lthoang looks like we've missed the import statement