support parameterized queries and prepared statements

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.
replace variadic with slice

When database/sql delegates to the driver Query or Exec functions, there
will be only one argument, a slice. The slice is not expanded to
variadic arguments. This change makes it clear that what the query and
exec functions will be passed will be a single slice of args, and not a
a slice with a single element that is a slice.
add SetQueryWithArgsFunc docs

* update README with section about SetQueryWithArgsFunc
* add SetQueryWithArgsFunc example
add SetExecWithArgsFunc docs

* update README with section about SetExecWithArgsFunc
* add SetExecWithArgsFunc example
Merge pull request #10 from bhcleek/support-args

support parameterized queries and prepared statements