support validation of slices of structs

[woodsaj]

Validation does not currently work for slices of structs.
eg.

type Person struct {
  FirstName string `json:"first_name" binding:"Required"`
  LastName string `json:"last_name" binding:"Required"`
}
type Group struct {
  Name string `json:"name" binding:"Required"`
  People []*Person `json:"people"`
}

sending a payload with the following is currently validated as correct, though it should raise an issue for the missing FirstName

{
  "name": "Group1",
  "people": [
    {"first_name": "anthony", "last_name": "woods"},
    {"first_name": "", "last_name": "invalid"}
  ]
}