Added initial Directory Structure docs: middleman-guides/issues#61

Hey guys, just finished [porting my site](http://sourcey.com/migrating-from-wordpress-to-middleman/) to Middleman and wanted to say thanks!

* Added initial "Directory Structure" docs which hopefully fixes middleman/middleman-guides#61
* Added Sourcey and source code to "Built Using Middleman"

PS. Wasn't sure about JP translation so I used English. Google Translate says Directory Structure = ディレクトリ構造 but it probably means something completely different ;)