undo no longer works with new ID system

[kfarr]

after this commit to enable erase feature: 84f5a2c

it was necessary to change to uuid when creating new objects, discussion here: #20

this breaks the existing undo feature which simply looked for the ID 1 less than the value of the total number of city objects (https://github.com/kfarr/aframe-city-builder/blob/master/lib/builder-controls.js#L250).

undo should be fixed to support the new uuid system