Relatively translate floating clients to the correct monitor

Here is a sketch of what I was trying to get at in #80. Basically, it translates the window from the origin monitor to the correct monitor preserving the *relative* corner offsets. In other words, leave w<sub>w</sub> and h<sub>w</sub> unchanged and let __a__/__b__ == __c__/__d__ (where __a__, __b__, __c__, and __d__ are vectors).
![geometry](https://f.cloud.github.com/assets/310393/1414024/dc199ff2-3e63-11e3-92cd-137c23a9fd48.png)

You'll probably want to merge `translate_client` with `translate_position`. I didn't try to do this as I don't want to break something else.