add options 'alwaysGrowRight' and 'alwaysGrowDown'

It can be useful to be able to generate sprites that only grow right or down. I needed that to generate a sprite that I can animate using css, like in this fiddle:

http://jsfiddle.net/simurai/CGmCe/

I added the options 'alwaysGrowRight' and 'alwaysGrowDown' to enable this behavior in node-spritesheet. Worked like a charm, so I thought I create a pull-request.

The thing is I never wrote any CoffeeScript before and there is probably a much more elegant way to make 'options' a class variable (see the change in builder.coffee, line 129+).