Fix for #80 - Fixed collision

I found a rather simple way to fix falling/walking through blocks.  It just ensures that the step count is large enough to avoid skipping over the 'pad' amount in the collide function.  It could probably look a little prettier, but I figured there was a good chance of a lot more changes happening to this function...

This is my first time submitting a pull-request, so hopefully I've done everything right...

Fixes: https://github.com/fogleman/Craft/issues/80 & https://github.com/fogleman/Craft/issues/36