[WIP] Support for #[sqlx(default)]

[OriolMunoz]

Closes #308

I'm not used to doing stuff with macros, but this seemed easy enough to give it a try.
Still needs documentation, more tests? (where?) and whatever fixes are necessary 😃

[mehcode]

This looks pretty good. Thank you for working on it. The FromRow trait documentation needs a bit of work to talk about the different attributes, see here: https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/from_row.rs#L8

If that's something you want to work on, please go ahead ❤️ . I'll go ahead and merge this though as its a nice bump forward. 👍