Fix/yml

There are two issues atm:

1) Importing a hash with nested keys example:

```
en:
  home:
    header: 
      title: "Foo"
```

This results in an error (invalid yaml)

2) Exporting the PhrasingPhrase records to yaml with nested keys can't be parsed by I18n
For instance a PhrasingPhrase record having the key "home.header.title" and value "Foo"

When exporting you'll get
```
en:
  'home.header.title': "Foo"
```

But if you try I18n.t("home.header.title") (which phrasing does when you first visit the page, to build the records) it gives an translation not found error. Resulting the value be set as the key in search_i18n_and_create_phrase

This fix, fixes both issues, by allowing a not-flattened import and export of the yml.

Special thanks to @brianweiner for the flattened method.