We should be able to pass a model to `update`

[sgrif]

I'd like to be able to write something like this:

let post = try!(posts::table.find(id).get_result::<Post>(&connection));

update(post).set(posts::published.eq(true)).execute(&connection)

post in that example should be a shorthand for posts::table.find(id). Why not just write that? Because I want this code to return Err(DatabaseError::NotFound) if the id doesn't exist. This will probably require a new trait, instead of tacking it onto an exiting one. I've been mulling over #[derive(Identifiable)] for a while, which I think will eventually be required for associations anyway.

[sgrif]

This was implemented by a737308