Fix a few things

While making this I noticed that the code style is quite messed up (http://dev.minetest.net/Code_style_guidelines).  but that will take a while to fix, and some reorganization is needed too.

  * Only load images once, rather than on every frame
  * Remove nodebox count limit
  * Add build files to .gitignore (and move things other than executable binaries out of bin/)