Unify imports

Now imports follow the following structure:

1. __future__ line: exactly one line allowed:

        from __future__ import (unicode_literals, division, absolute_import, print_function)

   (powerline.shell is the only exception due to problems with argparse).
2. Standard python library imports in a form `import X`.
3. Standard python library imports in a form `from X import Y`.
4. and 5. 2. and 3. for third-party (non-python and non-powerline imports).
6. 3. for powerline non-test imports.
7. and 8. 2. and 3. for powerline testing module imports.

Each list entry is separated by exactly one newline from another import. If
there is module docstring it goes between `# vim:` comment and `__future__`
import. So the structure containing all items is the following:

    #!/usr/bin/env python
    # vim:fileencoding=utf-8:noet

    '''Powerline super module'''

    import sys

    from argparse import ArgumentParser

    import psutil

    from colormath.color_diff import delta_e_cie2000

    from powerline.lib.unicode import u

    import tests.vim as vim_module

    from tests import TestCase

.
Remove unneeded # NOQA comments

It appears that something has changed and it no longer complains about double
declaration in else or except blocks.