v1.4.0 prep - DO NOT MERGE

This is not yet the complete 1.4.0 code, only the groundwork. There are a few more things I'll handle as separate PR's. 

- [x] testing under Python 2.6
- [x] code cleanup
- [x] test cleanup
- [x] fix two incorrect tests
- [x] don't use `armstrong.apps.content` for tests
- [x] drop Lettuce
- [x] README updates
- [x] update packages for currently supported Django versions (1.3 and 1.4)
- [x] be clearer about using Sorl
- [x] drop `django-reversion` requirement, which was never used

As additional PRs:
- [ ] Django 1.5 support is tricky because of deprecated User Profiles, which we use in one place
- [ ] deprecate the video stuff in favor of `armstrong.apps.embeds`
- [ ] slug field length issue :: armstrong/armstrong#43
- [ ] timezone support armstrong/armstrong#80
- [ ] related to timezones is #31 (though i believe this is moot)
- [ ] default URLs... armstrong/armstrong#28
- [ ] @tswicegood thoughts on #25? 

I think we should do two quick releases. **1.4** will tie up loose ends and be the last version to support Django 1.3. **1.5** will support Django 1.5+ and maybe drop 1.4 support (two reasons: `django-taggit` and the profile issue). Depending on how we solve the profile issue, we may need to break BC or at least break behavior. We can discuss more on the PR for that.