Autosave

Adds autosave for previously created posts
Adds previews for posts
Adds JS alert when leaving a page with unsaved content
Capybara has a feature that allows you to execute arbitrary JS. You should be able to utilize that to call your saveForm method manually. Something along the lines of:

```
page.execute_script "Storytime.instance.saveForm()"
```

To call it this way though, saveForm (which really I'd like to see be something a little more descriptive, like autosavePostForm or something) needs to be a method on the editor class. So the method definition needs a colon instead of an equals sign.

```autosavePostForm: ()->``` instead of ``` saveForm = ()->```