Added Showdown 0.3.1 definitions

I've added definitions for [Showdown](https://github.com/coreyti/showdown), the Markdown parser. Apologies for not signalling that I was working on this first. 

This branch is passing the Travis build and I've followed the contribution guidelines - however, if there any issues with it let me know and I'll be happy to fix it up.
please change to following code.
```
declare module "showdown" {
  export = Showdown;
}
```
Usually, showdown.d.ts is not in same folder as the actual code.