Delete generated MANIFEST and ignore it from now on.
Fix setup.py and directory structure such that `import human` works as expected.

I misunderstood the documentation, but this makes human behave
correctly when importing. The functions from_human and to_human should
be available as:

    >>> import human
    >>> human.from_human('1.5G')
    1610612736

or

    >>> from human import to_human, from_human
    >>> from_human('1.5G')
    1610612736