escapeString: convert special characters in a string into their escape sequences

Escapes special characters such as line feeds or tabs in a string and returns the escaped string. Useful for debug purposes.
Receives a string as a parameter and returns the escaped string that will reproduce the string when printing it with sprintf().
If you need to write the escape sequence using sprintf, double escaping by using
sprintf(escapeString(escapeString(str)))
or passing a single escaped string as parameter sprintf('%s',escapeString(str)) will do the trick.
Note that this function is not optimized for speed.