Updating README

Trying something crazy here. Feel free to push back if it doesn't match your style :grin:

You can preview the formatted readme [here](https://github.com/csexton/unite.vim/tree/readme).

I thought it would be fun to add a bit of a logo for Unite. I wasn't
sure what the best mascot for this vim plugin would be, but figured it would
be hard to beat a dragon. So, I went with a dragon, one who's body is in the
shape of a "U". I thought it came out pretty well, especially for a quick
effort.

![unite.vim brand](https://s3.amazonaws.com/github-csexton/unite-brand.png)

Added an animated gif screen cast of some basic unite functionality.
This seemed like a better way to get the basic concepts of the plugin
across quickly, so I dropped this in at the top of the readme to try an hook people's attention and get them interested.

![animated gif screens](https://s3.amazonaws.com/github-csexton/unite-01.gif)

Rearranged a bit of the content to group resources together in one spot.