Add version API

There are two ways this could be used:
  1. `minetest.version` - Efficient, could be added in `ScriptApiBase::ScriptApiBase()`.
  2. `minetest.get_version()` - Mods can't modify each other's copy it because each call gets it's own table.

Pro:
  * Allows for informational display of the version (useful for, eg, `/CTCP MinetestServer VERSION`).  In fact the main menu already has access to `get_version()` for the credits.

Con:
  * Mods could use the version to detect features, instead of using `minetest.features` or a simple existence check.  This patch doesn't include any version numbers, so mods will have to parse the numbers out of the version string, making it especially unlikely that they will use this method (and even if they do it isn't really our problem).

(And yes, there are no docs yet)