Add JSON support to RequestBuilder

[seanmonstar]

Using serde/serde_json:

// on RequestBuilder
fn json<T: Serializable>(mut self, json: T) -> Self {
    // sets Content-Type to json
    // sets body to serialized json bytes
}

Would allow:

reqwest::post(url)
    .json(SomeSerializableThingy::new())
    .send()

[seanmonstar]

This has been added. It could use some more documentation.