Open-ended recurring donation support (litle eCheck)

This commit adds open ended recurring donations support to the litle eCheck payment method. Specifically, it adds a setting to the gateway where the user can specify how many initial future donations to create (default 24). Each month when a donation in the series is processed successfully, a new donation is added to the end of the series and exported to Salesforce. Current known issue is that the "number of installments" field on the recurring donation object is not updated to reflect the new opportunity. This is a large issues that requires across the board changes to address.