[Java] Add authentication support (API key, HTTP basic)

[xhh]

Add HTTP basic and API key authentications to Java client, like what has been done for PHP and ruby clients (#765, #783), except that the authentications are stored in ApiClient instances rather than stored and accessible globally.

Some example code:

ApiClient client = new ApiClient();
client.setApiKey("special-key");

PetApi api = new PetApi(client);
api.getPetById(new Long(1));

For discussion, another global config approach might be (not implemented currently, pseudo code below):

Configuration.setApiKey("special-key");

then all ApiClient instances will use that key.