Changes for linux

Thanks for creating this module.  

Here are some changes to get it working on linux.
I haven't tried this in windows.

Add missing folders in lib/templ/xl/
Add support for unix
Fix example in README.  When you use the . folder to zip it up it'll create the xlsx file in the temp folder which gets removed afterwards.
Removes warnings in the latest verion of nodejs about octals, fs.existsSync