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
and a case to sting is missing - $l->t() is not returning a string but an OC_L10n_String

working code looks like:
```php
		foreach ($mailboxes as $i => $mailbox) {
			if (in_array(base64_encode($mailbox->getFolderId()), $specialIds) === true) {
				$mailboxes[$i]->setDisplayName((string)$l->t(ucwords($mailbox->getSpecialRole())));
			}
		}