Markdown Preview

@edsamson 
* Added lib/MarkdownWriter, utility module for dealing with markdown
* Added Api::UtilityController, to be used for miscellaneous API
  endpoints not related to any models
* Added new API endpoint to preview markdown parsed html

```bash
➜  eduki git:(api-lesson-preview) ✗ curl localhost:3000/api/utility/preview --data "body=apple"         
{"body_markdown":"<p>apple</p>\n"}%                                                                                 
➜  eduki git:(api-lesson-preview) ✗ curl localhost:3000/api/utility/preview --data "body=# apple"
{"body_markdown":"<h1>apple</h1>\n"}%                                                                               
➜  eduki git:(api-lesson-preview) ✗ curl localhost:3000/api/utility/preview --data "body=## apple"
{"body_markdown":"<h2>apple</h2>\n"}% 
```