Update gluon.validators.py to process a string of values submitted through a multiselect form field

This update would allow a string of values to be used to update a list:reference field through a form (e.g., '1,2,3,4' would be parsed by the validator into the ['1','2','3','4'] list format expected by web2py's DAL when updating a list:reference field). This update would require import and use of ast.literal_eval to transform the string into a list of values.