Adds lastlogin to auth file. Fixes issue #1604

Adds a time of last login to the end of each players line in auth.txt, and adds chat command, "logintime <name>" to get the login time of a player. If left empty the command returns your last login time.
This breaks backwards compatibility. A better approach would be:
```lua
local t = line:split(":")
local name, password, privileges, lastlogin = unpack(t)
if not lastlogin then lastlogin = 0 end
```
or some such.