Allow a specified `timeout` option for Net::HTTP

All methods can now take a `timeout` option to specify the underlying
`Net::HTTP` request's `read_timeout`:

```ruby
geocodio = Geocodio::Client.new
addresses = ['this', 'will', 'take', 'a', 'long', 'time']
geocodio.geocode(addresses timeout: 60 * 5) # Timeout of 5 minutes
```

This fixes #2

Signed-off-by: David Celis <me@davidcel.is>