Make use of flake8-import-order to ensure some additional consistency

Also fixes existing violators :-)

I'm not wild about the ordering of::

    from nacl import signing
    import nacl.c
    from nacl.exceptions import SomeException

But that's fine, a solution might be to remove the ``import nacl.c`` style.
Merge pull request #92 from alex/flake8

Make use of flake8-import-order to ensure some additional consistency