Fix #67: client.refresh without connecting

[serv]

#67

This PR supports this use case below. This use case is needed because you need to be able to refresh access token without an access token.

client_hash = {
  api_key: 'API_KEY',
  api_secret: 'API_SECRET',
  refresh_token: 'REFRESH_TOKEN'
}

client = Napster::Client.new(client_hash)
client.refresh # => returns new access_token by refreshing it