Provide `{To,From}JSON` instances for {5,6,7}-tuples

Providing this instances is motived by the Haskell Report suggesting
that libraries ought to support tuples up to a size of 7
(see http://www.haskell.org/onlinereport/basic.html#basic-tuples):

> The Prelude and libraries define tuple functions such as zip for
> tuples up to a size of 7.

This fixes #144