Big Building class overhaul (refactoring & bug fixing)

Refactored original crappy "my first THREE.js program" code.  Enlarged
block size to allow for 10px wide pavements and still leave 100*100px
for buildings.  Refactored building-parts positioning code and fixed
silly bug where City.width*City.depth (in *pixels*) buildings were
generated onload, instead of only City.width_blocks*City.depth_blocks
buildings; startup is now... somewhat faster. ಠ_ಠ
Added controls to display debugging axes (press "a") on keyboard
Adjusted helicopter flight path to avoid new (wider/deeper) block dimensions