ui-component checkbox_field fix

Updated the id of the generated hidden input so clicking the label toggles the state of the checkbox.

OUTPUT BEFORE 
![before](https://cloud.githubusercontent.com/assets/2614324/3176628/2b4a9e86-ec07-11e3-8ee9-1d9d35669b5a.png)
OUTPUT AFTER
![after](https://cloud.githubusercontent.com/assets/2614324/3176630/2d7bee4e-ec07-11e3-9093-52383adedb64.png)