Remove text marshaller/unmarshaller support. Use sql.Scanner and driver.Valuer instead.

@Babazka this PR reverts https://github.com/vmihailenco/pg/pull/3. The idea with TextUmarshaller interface looked nice, but in practise it causes more issues than it solves. One great example is time.Time, which implements this interface and breaks time parsing.

I suggest to use sql.Scanner and driver.Valuer instead. @Babazka does it work for you?