Update requirements; nix test-utils requirement

This updates the versions in the requirements file, removes unneeded
imports in test files and makes a small change so we don't need
test-utils anymore.
Move extract_tower_python and extract_tower_template

Moving these to the base makes it way easier to configure tower.
For example, instead of this:

    DOMAIN_METHODS = {
        'messages': [
            ('fjord/**.py', 'tower.management.commands.extract.extract_tower_python'),
        ]
    }

we can do this:

    DOMAIN_METHODS = {
        'messages': [
            ('fjord/**.py', 'tower.extract_tower_python'),
        ]
    }

I added an import to tower.management.commands.extract so that older
tower configurations continue to work.
Update README

This documents tower. There's probably some things missing, but it'll
make people (aka me) much happier.
Merge pull request #18 from willkg/updates

Updates