FEATURE: bulk selection in the admin post selection

Adding a button to select multiple posts by iterating the post stream and toggle them, which means the posts selected would be unselected if they are in the marked zone.

![1](https://cloud.githubusercontent.com/assets/297343/3634028/1e9e0750-0f10-11e4-9beb-a5631ed9d97d.png)
![2](https://cloud.githubusercontent.com/assets/297343/3634029/1ecdb874-0f10-11e4-9a26-b4be5ea77bc4.png)
![3](https://cloud.githubusercontent.com/assets/297343/3634027/1e742688-0f10-11e4-9a27-1b15ecbc3c03.png)

Regarding: https://meta.discourse.org/t/add-select-below-and-select-above-admin-post-selection-action/14905