Add async chunk generation using web workers

Uses webworkify to run the generation algorithm in an asynchronous
worker. Somewhat based on rachel-carvalho's chunk loading using web
workers in voxel-mars: https://github.com/rachel-carvalho/voxel-mars/blob/cf1dbedeccec09a4f266b2864180ec13ba749c17/client/index.coffee

With this change, chunk gen can now take an arbitrary length of time,
without lagging the main game thread at all (constant 60 fps).

Closes https://github.com/deathcap/voxpopuli/issues/25