Fix visibility functions on the borders of the custom grid.

#7 

in 767px which is the maximum width of the custom grid:
![selection_019](https://cloud.githubusercontent.com/assets/7369740/5066357/b46cb7ca-6e3a-11e4-97ea-98a4e58f727f.png)

And in 480px which is the minimum width of the custom grid:
![selection_020](https://cloud.githubusercontent.com/assets/7369740/5066361/c47f2bac-6e3a-11e4-84f3-93b52450b78f.png)