Use importlib instead of __import__

Without this change, bad things happen on my system when running ``optional_package('sklearn.linear_model')`` (``sklearn`` fails to import from ``..externals``, and then it looks like ``sklearn`` is not installed on my system).

/cc @matthew-brett @arokem 
https://pypi.python.org/pypi/importlib/1.0.1

Should we include that as a fallback?