New PayPal/Payflow API Functions & A Multisite Bug Fix

* New PayPal® Pro API Functions (`s2member_pro_paypal_rbp_for_user`, `s2member_pro_paypal_rbp_times_for_user`). See: WebSharks/Private#158
* New PayPal® Pro (PayFlow® Edition) API Functions (`s2member_pro_payflow_rbp_for_user`, `s2member_pro_payflow_rbp_times_for_user`). See: WebSharks/Private#158
* Bug fix on Multisite Networks related to User deletions and subsequent logins on child blogs. See: WebSharks/Private#145