Fixed encoding issues in the `EmailContactsAndUsersVocabularyFactory`.

The OGDS always return unicode because of our mysql connection string:

```
mysql://opengever:opengever@localhost/opengever?charset=utf8"
```

MySQLdb requires a “charset” parameter to be passed in order for it to handle non-ASCII characters correctly. When this parameter is passed, MySQLdb will also implicitly set the “use_unicode” flag to true, which means that it will return Python unicode objects instead of bytestrings (See http://docs.sqlalchemy.org/en/latest/dialects/mysql.html#module-sqlalchemy.dialects.mysql.mysqldb fore more informations).
Merge pull request #599 from 4teamwork/pg_vocabulary_encoding_fix

Fixed encoding issues in the `EmailContactsAndUsersVocabularyFactory`.