patch to give user the option of detaching tmux windows upon closing

Currently when closing tmux windows, the only options given are to kill or to hide the windows. This patch creates an additional "Detach" option that detaches the tmux session upon closing the window.

Previous UI:
![old](https://cloud.githubusercontent.com/assets/7845589/4908110/195ab692-6467-11e4-97b0-36cff80232b1.png)

New UI:
![new](https://cloud.githubusercontent.com/assets/7845589/4908119/2c83cfd8-6467-11e4-985e-c866f591e84e.png)