Crashing bug in replacePlaceholdersAndPrintStringFromBlock:

The method stringByReplacingOccurrencesOfString:withString: throws an
exception if the withString: parameter is nil!  Added checks to make
sure we have a value before trying to do the replacement.
Merge pull request #5 from stury/master

Crashing bug in replacePlaceholdersAndPrintStringFromBlock: