Added option to disable entity selectionboxes. (Minecrafters rejoice)

Adds an 'entity_selectionbox' option to minetest.conf, **false** by default.

When set to false, mousing over an entity will not display a selection box. (a la Minecraft)
When set to true, it will have the same behavior as normal.

Why is the default false you ask?
  Simply because setting it to true is ugly. Most people (as far as I can tell) think so, and those that like it are   probably more used to Minetest, and therefore know how to change their configuration settings.

Enjoy!