Raise OSError if `from_folder` gets an invalid dir

Now it's not possible to create an environment from a folder that does
not exist.
Adding a command line interface for Milieu

Now it's possible to use the basic read commands of Milieu from the
command line, by using the `__main__` module that works as a package
entry point in python. Basically, you just have to do this:

  $ python -m milieu -f <my-cfg-file> get MYVAR
New release: 0.0.6