New slideshow plugin and tests

I always find myself adding the same plugin logic for slideshows.

This plugin is an attempt to device a slideshow logic that's close enough to match everyday use of slideshows, i.e. for front pages where there are 3 slides, each with a picture, caption text, and a link. It does not contain any static files, though. As such, it does not run out of the box, but requires the user to override `cms/plugins/slideshow/slideshow.html`.

I've added a simple test. I hope you can accept this in the 2.4.x branch, as a new plugin will not be added to any project's INSTALLED_APPS.

![screenshot from 2013-07-01 01 46 27](https://f.cloud.github.com/assets/374612/728050/5f539404-e1df-11e2-98f3-e6bbe333ede0.png)

Should I add migrations?