Drastically reduce the amount of managed state in AMWindowManager.

Collapsed activeWindows and inactiveWindows into a single windows array that
just gets filtered based on AMWindow's public interface. This makes the window
manager a lot more straightforward and easier to deal with.
Add the ability to mark windows as floating.

Added a command key toggle-float mapped to mod1 + t by default. This also allows
for rotating focus to unmanageable windows.
Add a note about floating windows to README.
Merge pull request #32 from ianyh/floating-windows

Floating windows

Closes #27