Get gateway credentials options

Refers to #21

This PR adds extra data into `gateway_options` response. Since API provides information
on what fields are required per each gateway authentication mode, its useful to have it returned
too.

Example:

``` ruby
env.gateway_options.each do |gw|
  # get gateway auth modes, returns one by default
  gw.auth_modes

  # list required creds per auth mode
  gw.auth_modes.each do |mode|
     # list credentials
     mode.credentials.each do |cred|
       puts creds.name
     end
  end
end
```

Added 2 classes:

- AuthMode
- Credential

All tests are good.