Expand test suite

The current [test suite](https://github.com/bitwalker/timex_ecto/blob/master/test/timex_ecto_test.exs) contains a single test for `Timex.Ecto.Time.cast/1` when the input is a string. Other tests are likely needed here:
- [ ] `Timex.Ecto.Time.cast/1` when input is binary
- [ ] `Timex.Ecto.Time.cast/1`when input is a timestamp
- [ ] `Timex.Ecto.Time.cast/1` when input is a map like `%{"calendar" => . . . }`
- [ ] `Timex.Ecto.Time.cast/1` when input is time
- [ ] `Timex.Ecto.Time.load/1`
- [ ] `Timex.Ecto.Time.dump/1`
- [ ] `Timex.Ecto.DateTime.cast/1` functions [here](https://github.com/bitwalker/timex_ecto/blob/master/lib%2Ftypes%2Fdatetimetz.ex)
- [ ] `Timex.Ecto.DateTimeWithTimezone` `.cast/1`, `.dump/1`, `.load/1` functions [here](https://github.com/bitwalker/timex_ecto/blob/master/lib%2Ftypes%2Fdatetimetz.ex)
- [ ] `Timex.Ecto.Date` functions [here](https://github.com/bitwalker/timex_ecto/blob/master/lib%2Ftypes%2Fdate.ex)

I'm happy to put up a PR for these and others that you might think are necessary!