Remove HAML dependency in favour of installer-based dependency (like Devise)

Haml is a great template language, but for someone like me who's already using an alternate template system like Slim, it feels a bit awkward to require multiple template systems in production.

I assume existing Haml users might also want some more control over their Haml setup as well? They would already have `haml` or `haml-rails` in their Gemfiles. Moving the ugly mode toggle to the user's initializer also helps in the rare case someone might want to turn off ugly mode.

With the dependency removed, mavericks like myself who want to trim down their gem footprint can fairly-trivially override the `kaminari/twitter-bootstrap`, `layouts/rails_admin`, and `rails_admin` views inside their own apps.