Added a new field type 'Rating'

Please put the attached images in the directory 'assets->images->raty' as 'star-half.png', 'star-off.png' & 'star-on.png'. 

* star-half.png ![star-half](https://f.cloud.github.com/assets/772400/320921/c1525af8-997a-11e2-873a-0bfb7e70e225.png)
* star-off.png ![star-off](https://f.cloud.github.com/assets/772400/320922/c1e12170-997a-11e2-876b-069ae3af8b67.png)
* star-on.png ![star-on](https://f.cloud.github.com/assets/772400/320923/c1e81ea8-997a-11e2-99f5-913a750750c5.png)