Added new dub.json and compatible snippets

https://github.com/d-widget-toolkit/dwt/issues/12

You can open the dub.json in MonoDevelop Mono-D (http://mono-d.alexanderbothe.com) and the completion / IDE will work. I've added a snippet example in `snippets/dub.json` which compiles completely by itself when built with `dub run`

No linux support yet, but its dub commands are obviously targeted to windows and it shouldn't be complicated to copy and paste it for linux (except for the shell commands in `preGenerateCommands`).