support parameterized queries and prepared statements

I'm not quite done; I want to add some more examples and tests, but I wanted to start the discussion to see if this is likely to be merged and if there are any big changes that you'd like for me to make.

Add support for arguments to the custom query function.

Refactored stmt so it can be used to stub prepared statements.

Made the type of the first return value of conn.execFunc consistent with
conn.queryFunc: changed its type from sql.Result to driver.Result.

Satisfy driver.Queryer in conn.