Create a common interface for protocol bindings

Currently, it seems that WoT consumers have to know in advance the protocol binding of a Thing. If a Thing has both an HTTP and a CoAP binding, a consumer has to decide statically whether to instantiate a `TDHttpRequest` or a `TDCoapRequest` from each form.

Would it be possible to instantiate a generic object representing a request (or, in the WoT terminology, an _operation_)? Here is a suggestion:

```java
Optional humidity = td.getPropertyByName("humidity");

if (humidity.isPresent()) {
Optional form = humidity.get().getFirstFormForOperationType(TD.readProperty);

if(form.isPresent()) {
// here is the proposal
Operation op = ProtocolBindingFactory.bind(form.get(), TD.readProperty);
op.setPayload();
Response response = op.execute();

// the response's interface is generic, it may include properties of the Expected Response class and a 'payload' property represented as JSON, as current TDHttpResponse objects
}
}
```

Protocol bindings may register themselves to `ProtocolBindingFactory`.