Add minetest.get_version() which returns the complete version string and the major, minor and patch version as numbers

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.:
```lua
str, major, minor, patch = minetest.get_version()
if major == 0 and minor == 4 and patch >= 6  then
  -- version >= 0.4.6
else
  -- version < 0.4.6
end
if str:find("dev") ~= nil then
  print("dev version")
end
```