Added tabs dsl for organizing content on active admin pages

Added tabs to active admin to enable users to better organize content on different page (e.g. show pages).

This is the result:
![screen shot 2014-09-01 at 12 14 20 am](https://cloud.githubusercontent.com/assets/1386966/4105442/381113e6-31aa-11e4-9d4e-5099d91fc2c4.png)

DSL:

```
tabs do
  tab :overview do
    span 'tab 1'
  end

  tab :details do
    span 'tab 2'
  end
end
```

I figured I will have it reviewed before starting to write documentation for it. Hope we can get this merged quickly. It is a relatively small change, but really useful when a model has too much data on the screen. You can even be cool like github and display a preview for your html/markdown in a tab just like github does :).