Added exception safe string.format function.

I've added an Lua exception/error safe version of string.format() to lualogging to protect against the situation where a logging statement throws an unexpected error. In my experience, this is particularly important in a production system - I don't want a logging statement to potentially crash an application. Typically, the format statement is trying to format a variable that is 'nil' and throws an uncaught error. The substitution of an error "safe" version of string.format() should avoid that situation.

Please consider merging in this change. I believe it will make your logger more robust in a production environment. Please let me know if you have any questions or concerns.

Thanks,

Glenn