Code style to fit PSR and removed some unnecessary 'else' statements

- Code style was adapted to fit PSR;
- Removed some unnecessary 'else' statements to make execution flow more readable and decrease nested code. (@see http://refactoring.com/catalog/replaceNestedConditionalWithGuardClauses.html);
- Changed an 'if' condition into MongoLid::save() method to avoid call isValid() when '$force' argument is true;