Make a sweet API

Call all methods as bot methods:
```java
SendResponse response = bot.sendMessage(chatId, "Hello")
        .parseMode(ParseMode.Markdown)
        .execute();
```

Async
```java
bot.sendMessage(chatId, "Hello")
        .parseMode(ParseMode.Markdown)
        .execute(callback);
```

`execute(callback)` or `execute(onSuccess, onFailure)`?