Split JWT middleware

use JWT middleware and validators separately
Example:
- validator
```go
type Validator interface {
	Validate(string, interface{}) error // interface is Claims object
}
```
- middleware 
```go
func JWT(validator Validator, cf func() Claims) Middleware {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
```

