Add a Debug impl for <DB>Row

[mehcode]

As an example, debug printing a PgRow should display something similar to HashMap:

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?

[garasubo]

Looks like Debug impl for MySqlRow is already added in 0.4.x