Merge pull request #29 from Stebalien/check-for-fence-ratio

Check to make sure the fence ratio is specified
Merge pull request #30 from Stebalien/mention-fence_ratio-argument

Document the fence_ratio RATIO argument
Merge pull request #31 from Stebalien/arrange-after-kill

Arrange after killing a window
Handle _NET_WM_DESKTOP client message
Allow selecting window for bspc actions

Instead of always acting on the focused window, allow setting the
selected window by prefixing the command with
`--window-id my_window_id`. This allows better scripting of window
actions.

It also, by necesity, changes a few commands:
 1. `focus` can now be called with no arguments to focus the selected
    window.
 2. `drop_to` can now be called with `here` in addition to `next` and
    `prev` to allow dropping a window to the current desktop.