Issue 50 - User defined exit codes

Checkout tests https://github.com/abozhilov/slimerjs-exit 

For the Windows .bat file I'm not sure how to implement exit status. Don't know if it even supported. 
By doing this, we will not know when there is a problem with Gecko. I think it is better to keep the "official" exit status code, and read the exitstatus file only if EXITCODE is 0.