Fixed building texture-stretching bug

Merged geometries' faces keep their original materialIndex values, so if
you create three CubeGeometries (with materialIndices from 0-5) and then
merge them, all the faces will still be pointing at materials 0-5,
regardless of how carefully you define custom for each face!
Minor refactoring of index.html

Removed unused variables, pulled out all camera target definitions into
a single variable, added 'type' attrib to script tag (lazy!), etc.
Major refactoring of Helicopter class

Added proper Helicopter model, refactored geometry/lights so they're all
children of Helicopter.model and all translations/rotations are relative
to Helicopter's position, etc.