Refactor request_phase method.

The request params are already accessible in request.params so there is no
need to get them from the session (which is set after the request by omniauth).
https://github.com/intridea/omniauth/blob/deea8f23642734217fc8a91cfe9a09a69595d638/lib/omniauth/strategy.rb#L200

There is also no need to conditionally initialize options[:authorize_params]
and options[:request_params] as these are already set automatically by
omniauth-oauth.
https://github.com/intridea/omniauth-oauth/blob/43008e42b257d9b7c0b98122cf03ce14b021d782/lib/omniauth/strategies/oauth.rb#L16

This refactor will make it really easy to add new params in the future.
Refactor spec.

We should stub the request and not the session (which is set after the
request by omniauth)
https://github.com/intridea/omniauth/blob/deea8f23642734217fc8a91cfe9a09a69595d638/lib/omniauth/strategy.rb#L200

It also doesn't make sense to set options[:request_params] to nil when
omniauth-oauth is already initializing it as an empty hash
https://github.com/intridea/omniauth-oauth/blob/43008e42b257d9b7c0b98122cf03ce14b021d782/lib/omniauth/strategies/oauth.rb#L16
Add i18n support.

We can now pass the `lang` param to the authorization url to dynamically
set the language of the Twitter prompt:
/auth/twitter?lang=pt

The default language of the prompt could already be set in the OmniAuth
config with:
:authorize_params => { :lang => 'pt' }

Fix #45
Refactor image_url method.

Since options is a class variable, there's no need to pass it as an
argument.