Add a Debug impl for <DB>Row

As an example, debug printing a `PgRow` should display something similar to `HashMap`:

```rust
PgRow { 0: value0, "column": value1, 2: value2 }
```

I'm not sure how we should print something like `PgValue`. We could ask the type and try and match to print out something reasonable looking, falling back to raw bytes? Ideas?
