Seo presentation multilang

| Q             | A
| ------------- | ---
| Bug fix?      | 
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #26 
| License       | MIT
| Doc PR        | will be done in one at the end

I did change the source of the locale inside of the `SeoPresentation` model to the `DocumentManager` by passing the current document. This is the reason why i stop with feeding the model with the `SeoMetadata` and injecting the complete document instead. This will be very helpful to implement other title/description strategies (#23) later too.

I added a little fallback on the applications default language if the document's language isn't found in the list of default titles. 

If nothing was found an exception is thrown.

All stuff is tested.