Responsive design

This is by no means a perfect and complete solution, but more a step towards responsive design for ActiveAdmin. There are probably quite a few things that I have missed.

There has been some discussion about this in the past (mostly #1027 - which seems to me like overkill). I'm quite keen to see it done - not because ActiveAdmin is primarily used on mobile devices, but because it's useful to be able to occasionally change something when not at a desk...

I've added a `responsive` mixin, which defines styles for viewports with a width of less than `$active-admin-responsive-width`, and can be disabled completely by setting `$active-admin-responsive: false`.

Tables are made responsive using an idea stolen from http://elvery.net/demo/responsive-tables/#flip-scroll. As this could potentially upset a few people, I've also added `$active-admin-flip-table`, which can be set to `false` to switch this off.

Since these are mostly style changes... have a few screenshots!

![screenshot from 2014-05-24 10 26 50](https://cloud.githubusercontent.com/assets/1596233/3074232/9545fb88-e32d-11e3-93cf-b278bf25dfd2.png)
![screenshot from 2014-05-24 11 12 17](https://cloud.githubusercontent.com/assets/1596233/3074233/9546961a-e32d-11e3-9a33-8e454e99be23.png)
![screenshot from 2014-05-24 11 15 08](https://cloud.githubusercontent.com/assets/1596233/3074234/95488862-e32d-11e3-8045-046d12d3f15f.png)
![screenshot from 2014-05-24 11 16 00](https://cloud.githubusercontent.com/assets/1596233/3074235/95553b84-e32d-11e3-98a9-c1f2cb757279.png)