Fix Lua operator overloading

Check for NaN rather than null since that's what parseFloat returns
when it fails (see https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/parseFloat#Description)
Fix os.time in lualib.js

os.time should return seconds, not milliseconds
Merge pull request #27 from elisee/master

Fix Lua operator overloading and os.time.