Localize special folder roles

Fix for #346 
The english name of the special folders is a title-case of one of the following: 'inbox', 'sent', 'drafts', 'trash', 'archive', 'junk', 'flagged', 'all'

Of course this will need to be tested one the translations are available

Do I have to submit the strings to be translated somewhere, or can the extraction process automatically find them even though they're not string litterals?
-> https://github.com/zinks-/owncloud-mail/blob/localize-special-folders/lib/account.php#L290
The string extraction process cannot extract these strings.
We need to add something like this (yes - in a comment is enough):
```php
/*
t('Inbox')
t('Sent')
*/