Generate table by deriving Queriable

[defyrlt]

In issue #36 I was trying to get example with #[derive(Queriable)] to work and one of the things that confused me is that you still have to make a table! call and hand-write all the column types.
Here I'm proposing to make Queriable smart enough to generate appropriate table stuff or even table! call so you won't have to do it manually. For some cases you might want to manually set the type of the column, e.g

#[derive(Queriable, Debug)]
pub struct User {
    #[column_type = Serial]
    id: i32,
    #[column_type = VarChar]
    name: String,
}

P.S @sgrif thanks for all the great work you've done! I'm very excited about query builder/ORM in Rust.

[sgrif]

I do not like the idea of generating a table from Queriable, as it implies too heavily that a struct has to be one to one with a database table, and puts too much schema information in the struct itself. I'd prefer a macro which generates the table! call for you based on the database schema at compile time.

[sgrif]

Forgot to reference this from the other PR. First pass at this is #49 -- There will be an additional macro which loads table names from schema and calls that one automatically.