add screenshot shortcut key

add screenshot command

```shellscript
\#\!\/bin\/bash
timestamp="$(date +%Y%m%d%H%M%S)"
targetbase="$HOME/capscr"
mkdir -p $targetbase
\[ -d $targetbase \] || exit 1
import -window root $targetbase/$timestamp.png
```

see: http://awesome.naquadah.org/wiki/Screenshots
add tmux conf