Update to handle the new prompt parameter inside Auth Url

Hello,
As you can read at https://developers.google.com/accounts/docs/OAuth2Login#prompt a new `prompt` parameters can be use instead of `approval_prompt`

I have update google-api-php-client to offer the possibility to use prompt parameter by the use of a new setPrompt function (to be used with $Client->setPrompt() like we use $Client->setApprovalPrompt() today)

The param `approval_prompt` only allowed 2 options and `prompt` has a lot more flexibility as you can see here. Basically:

* `approval_prompt: 'force'` = `prompt: 'consent'`

* `approval_prompt: 'auto'` = not passing any value to prompt

And now there are a couple more options:

* `prompt: 'none'`: No authentication or consent pages will be displayed; it will return an error if the user is not already authenticated and has not pre-configured consent for the requested scopes. This can be used as a method to check for existing authentication and/or consent.

* `prompt: 'select_account'`: The user will always be prompted to select a user account. This allows a user who has multiple current account sessions to select one amongst them. Like this:

![GitHub Logo](https://f.cloud.github.com/assets/531168/662772/f35172ca-d75d-11e2-9d04-fbbdf0e02a89.png)

* `prompt: 'consent select_account'`: A combination of 2 previous options.
There are a couple issues with this pull request:

This explanation come from https://github.com/zquestz/omniauth-google-oauth2/pull/59