Added functionality to allow access to fields based on their tag names.

This is more of a rough idea of what I want to do. It imports two new packages. I can understand if you don't think this functionality will benefit your package. An example would be:

type A struct {
    Foo `json:"bar"`
}

a := A{
    Foo: "foo",
}
s := structs.New(a)                                                                                                                                                                                     
f, err := s.GetFieldByTag("json:bar")                                                                                                                                                                   
// do stuff with f