Implement body option

This option will allow to remove all code in `sent` and leave it only as API wrapper around `got`.

##### options.body

Type: `string`, `Buffer`, `ReadableStream`  

Body, that will be sent with `POST` request. If present in `options` and `options.method` is not set - `options.method` will be set to `POST`.
Because the way we have discussed is a better API for it and I don't like having two ways to do the same thing. If we keep it in for now, it makes it a lot harder to remove when we implement #16.