Fixed Plugin Metabox Field Width 

The input-element at the plugin metabox was displayed much too wide, while using the new backend design, provided by the ["MP6"-plugin](http://wordpress.org/plugins/mp6/)

Setting the input-field to **width=100%** fixed it and causes no harm with the traditional backend.

![bitlyfix](https://f.cloud.github.com/assets/1038789/565903/66300712-c64b-11e2-845b-846fb38576d7.png)