added test for callable Validator instances
added test for callable Validator instances

dkellner/cerberus,funkyfuture/cerberus,brettatoms/cerberus,pyeve/cerberus,nicolaiarocci/cerberus,calve/cerberus,MacHu-GWU/cerberus-sanhe,pyeve/cerberus,calve/cerberus,dkellner/cerberus,MacHu-GWU/cerberus-sanhe,funkyfuture/cerberus,nicolaiarocci/cerberus