Select locales manually and avoid saving empty translations

This feature adds the capability of selecting locales manually to the locales tab. Users can select which locales they want to fill out in the form. The locales that have not been selected will not be saved as empty translations. The selected locales can be removed, except the default locale.

Add a locale:
![add](https://f.cloud.github.com/assets/149311/1995761/def371c0-8509-11e3-915a-46552af8babe.png)
Remove a locale:
![remove](https://f.cloud.github.com/assets/149311/1995765/e7ba283a-8509-11e3-831e-975de16eddaa.png)

The implementation is Javascript based, I extended only the form builder to add a class to the link of the default locale.