Add min indices to grid sampler

The minimum number of the points in the grid.

You can ignore "too far grid" by setting `~min_indices` of GridSampler.

* `~min_indices=0`
![screenshot_from_2014-06-28 17 41 11](https://cloud.githubusercontent.com/assets/40454/3420254/387a59ec-fea0-11e3-89a8-e75db7c76ca8.png)

* `~min_indices=69`
![screenshot_from_2014-06-28 17 41 28](https://cloud.githubusercontent.com/assets/40454/3420255/387a9e34-fea0-11e3-87bd-6812e0384f51.png)