#26 extend property_string class with format method

[Dieter Adriaenssens]

add functionality to format/convert a string, f.e. a value stored in a database has a format (f.e. a date is stored as YYYYMMDD), but printing this value should be done in another format (f.e. DD-MM-YYYY), thus with a method getValueFormatted() you get the formatted value.
Using the method setValueFormatted() would convert the value back to the original format and set the value of the object.