Support Auto Renew and Recurring Subscriptions For Google Checkout

Using "Google-Handled subscriptions" type i.e level 2 integration - http://code.google.com/apis/checkout/developer/Google_Checkout_Beta_Subscriptions.html.

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Summary
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This issue adds support for following functionality for Google Checkout payment processor.
1. self-service and back-office creation of automatically renewing memberships, for configured membership types with "auto renew" option checked.
2. self-service creation of recurring contributions, for configured contribution pages.

When an auto-renew membership is created, a recurring contribution (payment) is submitted to Google checkout with instructions to repeat the payment on the membership end date. When google notifies CiviCRM site of a successful recurring payment, the associated membership record is renewed.

When fixed number of installments is not known / provided, an open-ended recurring billing / subscription will be initiated i.e. they will continue to renew until the member or admin staff cancels the auto-renew / recurring option .

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Implementation
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1. Update payment processor tables to have is_recur option set to true for google checkout payment processor. Fix upgrade script as needed.

2. Online Membership Signup and Renewal: When user selects 'Please renew my membership automatically' option or 'Your membership will be renewed automatically' message is displayed for a configured membership type.

3. Canceling Membership Auto-renewal

4. Online contribution page configuration: allows selection of Recurring Contribution properties when google checkout is selected as a payment processor.

5. Canceling Recurring Contribution

6. Wiki documentation updates to reflect that recurring is supported with google checkout and provide configuration instructions / link to requirements and merchant account config / setup.