Python 3 Compatibility

I had a quick go at adding Python 3 compatibility to Flower. If that's not a thing you want right now then feel free to disregard!

This doesn't add any new dependencies and passes all the tests on Python 2.6.8, 2.7.3 and 3.3.2. I'm not sure it will work on 3.0/.1/.2 but in theory supporting those is kind of pointless anyway.

`requirements.txt` is a bit annoying - Python 3 seems to want `anyjson` while Python 2 needs `unittest2`, which doesn't build for Python 3. For now `unittest2` is in `requirements.txt` as Travis will be upset otherwise.

Using `items` instead of `iteritems` in the templates might be a bit of a shame on Python 2 but I'm sure what any good alternatives would be. Probably not the end of the world though.