Add retries gem and catch common network errors

[benjaminjackson]

We should be wrapping all network calls to Fauna in a retries block like this:

with_retries(:max_tries => 3, :rescue => [Timeout::Error, Errno::ECONNRESET, Fauna::Connection::NetworkError], :max_sleep_seconds => 10) do
    self.client.post params
end

https://github.com/ooyala/retries