Setup field_description for meta field description

I have never contributed to a GIT project before so bear with me if this pull request isn't done right.

I tried to keep the usage as easy as possible. 

Usage:
```
$tests->add_meta_box( array(
	'id' => 'test-box',
	'context' => 'normal',
	'fields' => array(
		'test-field-one' => array('type'=>'text', 'field_description'=> 'Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor. Sed posuere consectetur est at lobortis. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus.'),
		'test-field-two' => array('type'=>'media', 'field_description'=> 'Aenean lacinia bibendum nulla sed consectetur. Sed posuere consectetur est at lobortis.')
	)
) );
```
Renders:
![field_description-screenshot](https://f.cloud.github.com/assets/5108034/1609401/c237bb60-5541-11e3-9678-54d79c560895.png)