Adds Floating functionality to mjolnir.tiling

This allows the user to specify if a window should or should not
be included in the tiling calculations and layouts.

[Here's a demo](https://dl.dropboxusercontent.com/u/113966/Screenshots/floating_windows.mov)

Please review @nathankot 
If a window is not included in the tiling calculation it won't be detected in the list of windows. Short circuit out of the method since there's nothing to do here.