Improved widget syntax

Allows to use widgets in a simpler way.

**Current**

```php
<?php echo Captcha::widget($this, array(
    'param' => 123,
))?>
```

**Proposed**

```php
<?php echo Captcha::widget(array(
    'param' => 123,
))?>
```

If you need to pass custom view object (should be a rare case):

```php
<?php echo Captcha::widget(array(
    'param' => 123,
    'view' => $myViewObject,
))?>
```