Implement query function

[MitchellWeg]

Implement a query function on the Connection object which allows the user to query for specific data.

query should return something (possibly a vec of the underlying datatype?). It shoud also allow the user to unmarshall it into a struct.

Example:

struct BarFoo {
   foo: i32,
   bar: String
}

for row in &conn.query("select foo, bar FROM foobar", &[]).unwrap() {
  let my_foo_bar = BarFoo {
   foo: row.get(0),
   bar: row.get(1),
  }
}