Move docstrings inside each method

Currently, the inline documentation isn't accesible

```
In [2]: mercadopago.MP.get_payment?
...
Docstring:   <no docstring>
```

This is a heavily used (and useful) feature to learn the API/SDK in an exploratory, interactive way. 

The reason is those strings before each method aren't, formally, *docstrings*. 

From the [docs](https://www.python.org/dev/peps/pep-0257/#what-is-a-docstring):

> A docstring is a string literal that occurs as the first statement in a module, function, class, or method definition