bugfix: implicit import

[vezeli]

I ran the example from README.md and it does not work because the implementation of the implicit import is wrong.

Currently, the problem with the implicit import of mle is that __all__ in mle/__init__ should be a list of strings and not a list of classes. See official Python documentation https://docs.python.org/3/tutorial/modules.html#importing-from-a-package for more info.

Before the fix:

>>> from mle import *
TypeError: Item in mle.__all__ must be str, not type

After the fix this works.

[vezeli]

Looking at Travis logs it does not seem that Travis is failing due to the changes made in this commit.