TTL, Unix Time, and more

This release adds Table.UpdateTTL and Table.DescribeTTL to modify and obtain time-to-live configuration (#51).
You can also use unixtime as a special struct tag modifier along with time.Time to force it to encode to Unix time in seconds, which is useful for TTL expirations (#109).

type Data struct {
	ID      int
	Expires time.Time `dynamo:",unixtime"`
	Value   int
}

Additionally, you can now specify custom idempotency tokens for transactions with WriteTx.IdempotentWithToken (#95). The docs have been slightly improved as well.
Thank you @roberth-k and @greggjs for the contributions.