Started implementation of rotated-cluster support.

-- Each key has a rotation angle, and center of rotation; available in
the editor.
-- Displaying crosshairs in the editor for the center of rotation.
-- Keyboard shortcuts to move CoR, and change angle.
-- Keyboard height is correctly calculated.
TODO:
-- Serialization is still wonky.
-- Can't click on rotated keys to select them.
Fix selection of rotated keys.

-- Marquee select just checks against the bbox, which is already
computed.
-- Single-click select transforms the mouse coordinates into key-space
by applying the inverse transformation matrix.  Then it can compare
against the two rectangles, just as it used to.
Updated FontAwesome reference.
Serialization for rotated-clusters.
Added a new ErgoDox preset.
Updated the README w/rotation info.