Add minetest.get_version() which returns the complete version string

minetest.get_version() allows mods to be compatible with many versions at the same time.
Not like fallback nodes if nodeboxes are not supported, more like small function that were implemented between two dev versions.
e.g.:
```
if minetest.get_version() == "0.4.6dev20130521" then
  -- minetest.some_function() is available
else
  -- minetest.some_function() is not available
end
```