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 
The window selection function was getting a little hairy, so I figured moving it out to a new function was a better approach.