introducing "star validation" for freeform validating

Validation spec now can have a '*' field that is defined as a function.
This function has the same signature as Backbone.validate() and it's meant
for freestyle validation like checking relationships among different fields,
calling a legacy .validate() function. This function must return: 
1. nothing/undefined if validation passes
2. if validation failed: an error string, a list of error strings, an error object.